Generally speaking, 28mm is great for unloaded road use, I'll go up to 32 for actually carrying stuff, and 35 if I think there might be a bit of offroad involved. I find it hard to resist dirt trails though, so most of my bikes lean to the wide side.

Then again, I just picked up some Fat Franks at 2.35 inches, and they are fun. I don't think I would tour with them, but I might consider it.