Is there a reason why you say they should be on the top? Most that I see, especially in newer houses, are on the bottom. The way a lot of cords are made today, especially extension cords, are for it to be on the bottom.
With the outlet properly oriented, the cord is less likely to fall out. You're less likely to touch the prongs by accident. Then, if the cord does fall partially out and something falls between it and the outlet it can short out and cause a fire.
If you look at the outlet packages, you'll see they're (usually) oriented with the ground pole up.
The NEC doesn't REALLY have a standard, but you'll notice it in places like hospitals or areas where sparks are a bad idea. Then again, the NEC is silent on all sorts of other things.
Also, a GFI eliminates that risk, as does a properly wired outlet which should just trip the breaker.
However, we tend to only mount a GFI when we're near water.