Yeah, I think it's tacky. If paper invitations were mailed to the guests, then someone has all their addresses anyway, so that's not an excuse. As TootsNYC said, if the hostess wants to do something nice for the GOH and save her some time, the hostess should fill out the TY envelopes, not make the guests do it.
I've never been asked to do it myself but I've heard it's common in some circles, so maybe if everyone expects it and even wants to do it, okay fine. To me it's one step removed from saying, "Grab a generic card that says 'thanks' on your way out, folks!" I mean, people took the time to come to a party and probably brought a present. I think they deserve more than a TY note with their own handwriting on it.