As far as I know continued bouncing would give the d20 more time to find its center, so it would be less random. That's pretty much the whole idea behind Gamescience dice. They aren't tumbled, so the edges are sharp and sometimes there's exposed mold flashing, but they stop rolling faster, meaning they're less likely to find their center
I don't know, but I used to play boardgames with a guy who would always try to do that. He'd hold the dice in his fingers so they couldn't move and would shake his hand, but the dice wouldn't change position. Then he'd just kind of drop them so they might not roll. He was successful sometimes in that it would give him a good outcome, but also there were like 4 other people around the table watching this, so we'd make him roll for real if the dice didn't move.
