Yeah, totally. And in addition, at the beginning of a show, a voice actress is sort of locked into whatever voice they decided to break out at the audition... even if they didn't expect to get the part and they didn't put as much thought into that particular voice as they wish they had.

I think some of the MLP actresses have admitted things along those lines. Once they get a chance to really develop their characters, though, they have more freedom to play with the execution.
Here's how dramatically Applejack changed from Season 1 to Season 2:
Pinkie Pie has been heading the opposite direction... actually, I guess it's that she's been kinda all over the place compared to the other characters. There was a period in Season 2 when Pinkie seemed to be settling on a more natural, conversational inflection that I really liked. But lately, outside her emotional focus episodes like Pinkie Pride, Maud Pie, etc., it seems like she's been getting progressively more shrill and screamy and it makes me sad.