I purchased an evo on Sprint that I'm going to root. I'm wondering, should I root the phone before or after activation on Sprint's network? Does it make any difference?
Do it after, just in case. Rooting shouldn't affect this process, but you never know. If it fails to activate you'll need to bring it in presumably, and if they see it's rooted they won't honor your warranty.
Installing a custom ROM can definitely screw this up, though, so I wouldn't do that. The comments on my answer to this question have some insight into that experience (fortunately he got it fixed).