Head tilting is a major part of an appealing pose. That's what I think. Of course, you don't tilt your head aggressively all the time!! Just a little bit, or maybe... a little more than just a little bit... you can find tilts in pretty much any pose. I'm just observing here, this is not a recipe. I don't usually like what comes out when animators follow too many recipes instead of focusing on real performance/acting.
There are many reasons for tilting your head, and perhaps one of the most common is the need for balance. In cartoon animation especially, many poses are drawn to follow a round C curve or S shape, and it's called Line of Action. In the rat below you can draw a smooth curve along his spine and head and legs too. The head tilt here is just a natural continuation of the body's line of action (btw, notice the rat's right hand looks kind'a stiff, hehe - this and the following rats are taken from the Ratatouille teaser and trailer).
This cartoony, graphic principle, goes actually hand in hand with a real world phenomenon - we balance our heads and bodies all the time. A twist of one body part can generate another twist of another body part, in either the same or opposite direction. Balance is achieved through opposites though. Look at this guy (this and the 7 following images... not containing rats... are taken from the excelent movie - Little Miss Sunshine):
His head seems to want to stay straight while the body twists, creating an arc, like an inverted C shape. This is what it seems at first glance, but we'll be back with this example in just a sec. Because it's actually not the need of the head to stay straight that we're examining here, it's the contrary. The head does need to stay straight so it can get a clear picture and function in optimal conditions. But its "straightness" is not that... straight all the time. The head needs to stay straight, but it often finds itself doing otherwise. OK, so from here on I'm only talking about tilts. Tilts occur in so many cases that's just ridiculous... but most of them are fairly subtle. It's almost like saying - the head is straight... but in effect it's not. I've given examples of more extreme tilts in this post though, just to emphasize the fantastic appeal of head tilts. Oh, and I'll only show left-right tilts, which I consider more special and appealing (backward-forward tilts are pretty basic, and everybody is using them all the time - well, OK, one thing about backward-forward tilts - I love how the Muppets keep shaking their heads all the time, now that's a brilliant caricature of real head movement, typical for the puppets-animation medium, but just... unique in that specific way it appears in the Muppets).
Look at me, I'm so tilted...
Notice how both characters are tilted towards their right.
Yeah, I'm looking. And guess what. I can tilt left too...
From the same shot as the first image - notice the line of action has changed from an inverted C to a C shape, the head tilting with the entire body from right to left (I always refer to the character's left and right).
Look at us tilting an twisting... aren't we cute? (Notice how their bodies are tilted towards each other, while their heads are tilted in parallel, towards their right)
OK, if you think this isn't such a big deal, let's look at real world examples, where you'll see that tilting heads are actually really going wild (it's weird that a cartoon doesn't use tilting more than real life... Could it be because the artist's eye has the tendency to straighten up and smooth out things...?)
OK, look at these guys and notice how tilted their heads are...
And then how they keep tilting from left to right
And btw, the kid in yellow is not talking, and he's kind'a stiff and ignoring the rest of the table. Yet... he does change his pose quite a bit. And he tilts with (or against) the rest of the group, hihi. They're like trees swaying in the wind...
Back to the guy in blue, and his family. Notice the variety of tilts :D
OK, and now look at the blue guy again:
Is his head tilted or is it his body? Well, in the above image, his head is almost straight, while the body is tilted. But actually as he stands up and walks, we see that his head keeps a similar angle with the body, and becomes tilted. Hihi. It's part of his current attitude.
And here... the whole gang seems to want to be tilting right (maybe it's the road...?). Yeah, except for the guy in white who's so stiff (he's got his reasons allright) - so maybe it's not the road...
Let's go and see more tilting examples. (This and all the following images are copied from various internet sources, if anybody is offended, please let me know and I'll take them out. Oh, and please don't sue me, I'm one o'them good guys...)
Why do people tilt their heads left-right?? Weeeeelll, maybe they need to fit in the picture...
Maybe it's their job...
Maybe they're trying to impress...
or to relax...
Kids do it a lot... all the time...
Even at an early age...
So why shouldn't divas do it?? They know it's sexy and appealing. Especially women!
OK, now maybe someone's trying a bit too hard here... look how tilted this head is... craaaaaazy :D
So women I guess were the first to realize the head tilt looks really good on a resume...
And got to real extremes...
While men were soon to follow... slightly more serious...
Tilting your head is good for your health too...
...aaand in case you need to scratch like a monkey... now that's some tilt!
Dogs do it... the love doing it!
And other animals too (even if it's sometimes a disease - yeah, poor fella', he's actually ill, looks like some animals have this disease that's called... head tilt, weird, huh?)
Some people do it sometimes in pairs...
It's being done successfully in other countries as well...
You wouldn't think this is much of a tilt, now, would you...?
...but look again (actually this pencil is a little off, and the tilt is slightly more pronounced).
Well, OK, finally, if you don't know how to do it (hihihi), here's a scheme...
Edit:
Along the same line, things should generally be not too straight in a pose for them to look natural. Here's an image from someone else's blog...
that underlines this clearly (although I think he meant "unless the pose calls for a straight... line of action" or something like that). Of course, there'll be times when you need a pose to be rather straight, uptight, but that should have a solid reason in the acting context. For natural, live, appealing poses, lean, twist, bend, tilt... just don't overdo it, don't completely mutilate the character :D The hardest thing in a pose is balance. But it's also probably the most beautiful.