3-9x is the most useful, i think. 14x might be the ticket for pure bench/varmint applications. speculation, though. 9x is the most i've used.
Top end mag I have had is 32x on a nightforce. Well worth it for BR stuff. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
Given the size of the targets (read: gophers) I think the 14x will be a good level. I have a 4.5-14 on my .22 right now and enjoy it. I just can't bring myself to pull the scope off the .22 for the .17 though.