I confess I can't think of a reason to decline a pre-snap foul from a competition standpoint, though there may be something I'm missing. While it's true that there is an option, I've never seen a pre-snap foul declined, and would be willing to bet that nobody has seen it more than one hand's worth of times.
FWIW, our association tells us to just give the signal and march it off, if necessary, where it's an obvious choice.