Doc - the instrument is based on a stops open-shut philosophy. Anything in between is not the intention of the design and although you may consider this a 'harmless' deviation I am still calling the police.
I asked Marc Savoy this question, can playing with the stops partially closed cause any damage to the reeds. He said no, that it's fine to do that. Jesse Lege also showed me how he does that sometimes to get a wet sound out of a dry tuned accordion. I've also done it myself and haven't noticed any ill effects.