Perfectly level is good in the direction 90 degrees from the direction the combs are running (if the combs are lined up North - South, then you should have the hive level from East to West). Tilting a little North or South, in this instance would be acceptable. I have combs with bases of wired beeswax foundation, combs with bases of plastic foundation, combs built by the bees without any foundation, and some man-made plastic combs. There is a variety of these types in almost every hive. For the sake of the foundationless combs, I carefully level my hives perpendicular to the direction the combs are aligned so that the bees aren't confused about where the comb should end (attached to the bottom bars).