I think you're right, Eddy - the bed does not have to be level to micron accuracy to work OK. However, it's just easier to aim at "level" across and along it than to get to take out twist by making it the same amount off-plumb all along. As to accuracy required - and I'm thinking out loud here so no guarantee of logic! - then a bed that's as about as wide as the centre height will need to be levelled to about the same accuracy as the lathe could be expected to cut. Say, 1 thou in 12" would be pretty good. That's 1 in 12000, which is about 0.005deg. That's a pretty good level...