Upload & Sell: Off
And when you get to doing 'volume' at events: 200,000+ photos per day, you'll find that shooting at 4-5Mpix enough.
Like John and glort, (and I'm sure you too, Forrest) we make sure to fill the frame with the subject - ready to sell and ready to print.
At 4Mpix with minimal crop, we can print 24x36's all day long - but that's a rarity, with common orders being roughly 30% 5x7s, ~4% 8x10, <1% anything larger and the other 65% images on disc.
As mentioned, larger file sizes do a couple things for you: take longer to download and use up more space to store... that's about it.
With over 20TB of images - mostly from just the past 2-3 years, I've yet to have a customer complain that any image doesn't have enough resolution.