I have made so many forms testing design that I rarely put anything on a form except wax. That said, it wouldn't hurt to put something on it to smooth out any minor imperfections.
With the router method, I only sand with a long curved block a little and wax the heck out of it.
If I were gonna keep a form and use it more than a few times, I would put a pc of .050 glass or formica on it.