I don't want to scare you but poo build up on the fluffy butt can lead to flies. Flystike may be rare but is real and deadly. It does extensive damage to livestock in Australia and New Zealand (mostly sheep) in particular and is most common in warm climates.
Flystrike is where the larvae of the blowfly hatch in feces stuck to a chicken's butt or in an open wound and begin to feed on the skin tissue of the chicken.
They will eventually burrow into the body and start eating the chicken from the inside out. They can tunnel through the skin and organs and will eventually kill the chicken.
Not to mention the flies can transmit salmonella among other diseases.
This is why I like to keep the butts clean. It's not common here but why take the risk.