Actually I'm not aware of any rules (unless you are in the military) as to which way a belt must go. For a standard belt I would think that whether you are right or left handed might make a difference as to which way it went.

So for a kilt belt where you have extra I would say what ever is more comfortable. For a kilt belt with a waist plate it doesn't matter, it will look the same either way unless the waist plate's design needs to go a certain way.