To answer your question, Card, it's a bit of a mixed bag. On one hand, the Styles are meant to coincide with the type of abilities you commonly use. On the other hand, their actual use in the RP will mostly be just be for dramatic effect (ie going into a Style-change as a result of character development and the like).

So yeah, a mix of those two methods. I mean, it wouldn't make much sense for a person using mostly Fire spells to suddenly go Diamond Dust, right?