I agree with the comments about the purpose of 'levelling' being to help maintain the accuracy of the machine by ensuring the bed isn't twisted. However I'd be careful with using the word 'level' as to me that word imply relative to the surface of the earth, which is a somewhat arbitrary reference (at least in this situation). So long as the bed isn't twisted it shouldn't really matter if it's tilted one way or the other, within reason. I guess it just happens that adjusting to 0° on your engineer's level is convenient...

Also bear in mind that if you're only ever going to be cutting fairly short parts on the lathe then you wont notice errors in diameter due to bed twist, so get it close but don't worry about it too much if you don't need it.