I think of restoration as just that, restore to the original condition. Don't retouch unnecessary things just to make it pretty. I try to keep everything in the original style of the photo. One restoration I did for practice, had a watercolor background. I tried to keep the restored areas in the same style. It's impossible to anticipate what is important to the owner of the photo, and I don't want to change something that would impact their memories. Now, having said that, I don't have a problem with reconstructing an ear, replacing a hand, etc. If I need to borrow from other photos, I will, but I want those parts to blend, so that you can't tell that they were 'borrowed'. I also believe in improving contrast, color casts, etc. while doing restorations, but I don't want to totally replace a background if I don't have to.
I guess it boils down to I try to restore, not retouch.