I have shared a printer with Vista from XP x64 Edition and it worked well.
Windows 7 downloaded the printer software through Windows Update and it works. I never shared one with W7 from XP but I would hazard a guess that it'll be fine. Vista and W7 both seem to network very well.
Plus W7 shares with XP just fine for me. The only problem was the drivers but I explained that in my earlier post.