I have never washed any of my belts.
However I was never told not to.
I just figured on the following reasons.
1. Traditionally there were no belts, but wariors used to have a (ohh man I cant remember the word for it.. I dont think sash is the word neither but it maybe,..) Anyway they used to wipe there hands on it and it would get dirtyer over periods of battle. Evidently you could tell how experienced they were by looking at their "sash?"
Thats the first story I was told.
2. I don't want to see what happens when a belt tangles around the spinning thing in the washing mashing.
(Yes Spinning Thing!, Im relly at a lose for words today.)