I just wear my belt into a pliable state... I must admit I too am a little to old school to wash my belt, but I certainly don't let it sit gathering mould or the likes; I take it out and air it between classes.
20 years of combined MA training and I haven't had an issue with a belt getting too nasty (mind you though I've never had one belt long enough to wear it out (Aikido might change that though
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Simple wear and time does wonders for making a belt easier to tie.