I think the rules say a downed oppponent is all four points on the ground (but I'm not 100% sure.) Riggs was definitely up.
Arlovski holding the fence was "Cheating" if you will, but Cruz was indeed clinically dead from the loaded right hand Arlovski waited on Herb Dean to let him throw (*which was f'ing hilarious.)
Now for holding the fence in terms of blatant illegal use of it, would be Florian against Sherk where Kenny held on for dear life. Luckily Sherk still got him down. Or the fight with Sylvia where he held on the fence to avoid getting slammed.
Holding on to the fence while GnP isn't a huge deal, Cruz was never rolling out of that and the ref has his eyes on the fighter to make sure he's alive, not the fence. But usign the fence to avoid getting slammed on your ass, that's a little lame. They should have 10000V on that fence when you grab it, you'll learn.