I don't personally see any particular need to clean unless there is something to BE cleaned. If there is, if it's a bunch of dust that has gotten worked into to all the little irregularities of the painted wood surface, a damp rag does it pretty well. If there is squirty poo graffitti-ed across the wall, warm soapy water and a big of scrubbing works. If the roost or fronts of nestboxes have gotten kind of grody, warm water (possibly soapy) and a bit of scrubbing generally works fine; for really dried-on stuff, you can lay a barely-damp rag on the area for a few hours and after that it is far easier to scrape or scrub off; for a SUPER nasty roost you can sandpaper it if you're desperate, although a good dustmask is a smart accessory.
JMHO, good luck, have fun,
Pat
JMHO, good luck, have fun,
Pat