With Maillons I'd suggest that screwing them down has more going for it than screwing them upwards - but only by a midges. When they're shut they are shut... period (if they aren't done up, as has already been noted they bend very easily - so done up is done up). However if the vibration can open the sleeve on a crab then it may on a Maillon, and since the thread is vital to the strength, then that's the clincher for me.
With Crab's I'm now thinking if it's an issue at all. Cave Troll suggests that with the locking sleeve top most, the rope is less likely to jump out. But how is that to happen? The gate would have to be open first and what sort of permutation are we talking here for that to happen (I'm not disagreeing, just wondering)? Also if it did happen that the gate opened why would it be any better that if the Crab popped off? This said Crabs do vibrate when loaded with someone ascending/descending and often the sleeve doesn't hold the gate closed - and I've witnessed this when Crab's have been done up both ways. If the conditions prevailed for our rope hopping out scenario to happen then it's going to happen anyway - isn't it?
Are we not in danger of seeing reds under the bed here? If a Crab is loaded correctly then there is very little chance of them coming apart even when the sleeve isn't keeping the gate shut. Maybe we should take a leaf out of our climber cousins and just use stiff sprung snap links
A bigger bug bear is people who pass a rebelay on ascent, kick the lower rope all over and the leave the Crab perilously at an angle in the bolt just right for cross loading or torquing open!!