My basic rule of thumb, a good smooth 1/8-1/10th surface error; no edge hook. General viewing, it'll kick butt on 99% of what you want to do. Try to keep any variance in the outer 1/4" of the secondary's aperture.
Planetary or other hi-rez/hi-x stuff, get the best you can possibly afford. (big apertures will put you several hundred $ in the hole.)
I tested one of Jay's secondaries when he was having trouble. Pretty sad!
Mark