Flea prevention?

i live in northern nevada and our unique climate means we have no fleas or ticks which is sweet. i've spoken to many vets who all say they have never treated a case of fleas in my town and i hear that reno (200 miles away) doesn't have any either but i work at wal-mart (while attending college) and i sell tons of flea collars and things so i'm wondering if its healthy to treat something your dog doesn't have? is it a preventative thing? i don't think i'd treat my dog for something he doesn't have.