I believe washing is part of a required maintenance schedule.
A GA aircraft must be washed (might be just the engine) when it has an anual or a 100 hours. Obviously, they can be washed more often as needs be.
The airlines have a similar rewuiement, but offhand I don't know it. Different sircraft get more dirty than others, like the Classic 737 as opposed to the NG 737 for instance. This is because the reversers slide on the NG a/c, but are the bucket type on the Classic model. The buckets direct a lot of dirty exhaust directly at the fuselage.