Generally when I work with new belts, I'll let them practice on me some, and I'll give them taps. They know that I'm giving it to them because I'm explaining as we go, but still, most white belts are absolutely elated that they got a tap, and also they learn alot about positioning and what to look for during a "real roll". I do this because I've been the new guy in class before, and I know how frustrating and just generally discouraging it can be to go 2-3 weeks without getting a tap. After I give them a few tips, I'll usually go with something like "Ok, let's really try now, ok?" The white belt is amped, and they really give their all, usually I'll let them get to a point and say "ok good, now when a guy gets you here, you can do this." and then move on to something and catch them.
IMHO it's sort of asshat-ish to tap white belts just to get taps in. I try to teach them what I know in hopes that they'll improve and I'll have another quality partner to roll with somewhere down the line.