Oh, I don't think of it as practice, but as legitimate writing that I've done. I've done fan work and I've done pro work (I actually did pro before I did fan), and I don't see why I should need to treat them differently--they both had the same amount of devotion spent on them, and if I'm writing a query letter about something, I don't know why I should be able to cite my creds as my one pro novel and several book reviews, but can't point out that I'm also the author of several well-reviewed stories that appear on _____ (wherever). I don't like treating some of my work like I'm ashamed of it, while I treat other work like it's legitimate just because I was paid for it.

Now, I have become a better writer through my fanfic. That's a given. Not because it's practice for pro, but because it's writing, and the more you do of it, the better you'll get. But I can't cite anything I've done in the last five years or so, because it's all fan work, and I think that's silly and insulting to fanfic, since it doesn't treat it as legitimate writing experience.