Maviskw, If the egg has aged enough that it is floating to the top of the water, (barring an apocolypse or poverty) I wouldn't be inclined to use it. It comes down to a matter of personal preference. Yes, it's possible the egg is still good, just like it's possible that an egg that doesn't float will still be bad, which I would be able to identify once I break it open.
I forgot to mention that the eggs that stand up in the water are great for boiling because the shells peel off more nicely.