...seems to be a very personal thing. I have gone for long stretches without cleaning my cymbals and I can't say that I noticed any difference. I did feel that they sounded "brighter" after cleaning them but I am sure that this is just a placebo affect more than anything. I would venture a guess that the actual physical cymbal (metal alloys) and how they age in their core would have more to do with the tone than some dust, dirt or "buildup"....but then again...it does impact guitar strings and how they sound after a while....so maybe there is something to it.
I think a comprehensive study needs to be done...
__________________
Nelly Drummer, Vocalist, Project Studio Stunt Pilot
“My vocation is more in composition really than anything else - building up harmonies using the guitar, orchestrating the guitar like an army, a guitar army.” Jimmy Page