I try to do that in my fics as well, simply to make it less "kiddy". *glances at your avatar and remembers an example* I've had a Buizel in one of my fics do all these things; growling, raising fur when aggressive, and other things too. In fact, this said Buizel had very otter-like behavoir, making "chuttering" sounds (that were supposed to resemble its in-game cry) along with saying "Buizel", fighting with members of its wild "clan", and so forth. Likewise, an Arcanine in the same fic barked and snarled in loyal dog-like defense of its trainer.
While it may seem weird, I find that giving Pokemon animal-like traits and behavior makes the fic both unique and more interesting. It helps it stick out, because the Pokemon aren't just like humans who say their name and have different bodies/powers. Instead, they seem to have more instinct and actual different kinds of mental consciousness, if you know what I mean... I'd do it if I were you, simply for the sake of making your fic more unique. ^^