My general procedure to clean a B board without a battery is:
- remove the PCB from the plastic shell
- use canned/compressed air to blow dust out of the shell and off the PCB
- wipe the shell down with water and paper towels (I try to keep the stickers dry, but they seem waterproof)
- clean any crevices in the shell with water and q-tips
- clean any dirt off the PCB with q-tips and rubbing alcohol, drying with compressed air
I'd avoid dousing the PCB because of the battery, but I dunno. Is there anything specific you're concerned about? The PCB will be inside the shell, does it need to be pristine or would blowing the dust off and cleaning the shell be good enough for you?