I've done it, and gotten away with it, but I don't make a habit of it. But like they said you might as well ride it until it blows.

BTW--you know you can still do offroad with slicks on your bike? Be a little careful until you're used to the handling. The first time I ever got slicks, I slipped going slow on wet grass and fell over. But after some practice I could ride on many surfaces with slicks, including wet grass. A MTB with slicks is a very versatile bike and IMO very good for city commutes if you don't feel much need to go over 16 mph or so. Hang onto your knobbies for winter riding.