Actually, there in the US, there IS a legal requirement to wash the aircraft prior to maitenance. When doing an annual or 100 HR FAR Part 43, Appendix D:
(a) Each person performing an annual or 100-hour inspection shall, before
that inspection, remove or open all necessary inspection plates, access
doors, fairing, and cowling. He shall thoroughly clean the aircraft and
aircraft engine.
Now, I'm not sure how this translates into airline maintenance (part 121 ops)... Now, of course, whether this reg is put into practice is another issue altogether!!!!
Cheers