My personal opinion is that unless a character is winded, or scared or there is some other dramatric purpose to highlite them breathing that animating breathing is a bit of overkill. Just watch a movie with live actors and see how much you notice them breathing in a normal talking scene. Not much. all the other little touches help though. Fiddling with something in the hands or the way a character balances. If you can afford to spend the time plussing a scene go for it, but don't pull your hair out doing minute details that nobody will notice. The Traditional Disney approach to animation that most US animators seem to follow has characters flouncing around like crazy in every single frame... I hate it. I prefer the Japanese approach which is more sedate and IMO more realistic, but can be hyperkinetic when it pays most, in the action scenes.