ralphmcpherson
Senior Master
It also looks wierd when a black belt doesnt know the forms/katas in a system where they wear a black belt.When in doubt offer to wear the white belt, and just do it, don't ask. If they want you to wear your old rank then they will let you know. I have done this at many schools and most of the time I am told to put on my actual rank.
There is also a perception thing, it looks weird if a white belt is beating (or holding his own) against black belts, it makes more sense to recognize that previous rank if that is a concern.