For the love of all that is matrimonial say yes first!!!
Then take him to dinner or a ball game or a duck blind or some other thing he enjoys and bring it up. If your first thought is; "OH MY STARS AND GARTERS, THIS RING IS HIDEOUS!!!" you may not have found the right guy. Or you could just lose it / swallow it / run over it with your car with the intent of getting all weepy and subtly angling for a new one. Admittedly, this is more than a little harsh. Have it appraised first. And don't get caught.
I'm assuming (and that's a big assumption now-a-days) that the guy surprised you and all that classy stuff. If you knew when the ring was coming down to the millisecond things are a little different.
Heh, looky there, my ex-wives taught me something after all.