This has been discussed a few times. Many usually chime in that their belt never needs washing in the first place. I came across this link, where the poster actually developed mold on his!
http://www.karatethejapaneseway.com...?t=10011&sid=3b0b77ed8e34b2aa797acc43a2a19cea
There are all sorts of old wives' tales, regarding the "evils" of washing one's belt.
Those tales are simply that; nothing more than fairy tales. If someone chooses to believe in such, that's his choice, but I'd much rather someone not actually believe that we can turn pumpkins into full-fledged, luxurious stagecoaches.
There are some people who refuse to wash their belts for the fear of "washing out the power," and to that I ask "does the belt itself contain all of that wonderful martial arts knowledge, or does the knowledge dwell within your own body?" I would think that regardless of whether you're wearing an obi or not, that you are still a martial artist, and that your knowledge is still 100% intact.
Besides, when someone smells badly, it creates a distraction to the others in the dojo. It's no different than someone who literally douses himself / herself with massive amounts of cologne / perfume, and makes people's eyes water.
Is it OK for someone to put his own personal pride, above the well-being of the entire class? I would think not.
Washing your belt in a gentle cycle in the washing machine, in combination with a mild detergent (Woolite works well for those who want to be as gentle as possible), combined with a very gentle spin cycle, and a hang-drying, will not harm your belt. Your knowledge will still be intact.
The only case where I can see someone not wanting to wash the belt in the washing machine, is if the belt were a very, very delicate one. In that case, immersion in a container of water mixed in with a mild soap (such as Ivory) and some gentle agitation can help remove the bad smell.
If, however, your belt is in such a bad condition that it could fall apart with even the above gentle treatment, then perhaps you should consider retiring that belt, and getting a new one. Again, your knowledge is within your own body, not within a piece of cloth.