I don't do it, and personally, I think people who do it all the time win the douche award for the match. People hate the Halo 3 community and we dare ask why, ha.
Now, I say "people who do it all the time" because there are moments when it's more acceptable; if you've jumped down and assassinated someone ten times in a row from the same spot, tea bag. If you kill them with a pistol and they had a rocket launcher, tea bag. You get the gist of it.