OK, as Roger said, they don't need to be perfectly level. Half the time your aircraft is not going to be level! You do want to be sure, though, that the carbs are securely seated in their rubber sockets, and that the O-rings and sockets themselves are in good condition. If any air is leaking past these, whether through damage, or just looseness, that is a bad thing. And, as rubber parts, the sockets do need to be replaced every five years at least.
