The only issue I can think of, aside from simply not needing them if you're tight enough, is that they could give the impression that the prop is tight enough when it's really not. Then instead of spinning the prop or throwing the prop on startup you do it at high RPM.

Downunder mentioned making a washer out of sandpaper to add friction, but I have something even better. Get some open mesh sand cloth from the plumbing section in your hardware store. It's rough on both sides, so any two things you clamp it between will not move. Since friction is what actually holds your prop in place, the open mesh sand cloth really makes a difference in how much torque you need.