Washing up liquid is corrosive. Never use it on a plane.
That said, I don't know what is best. I pay a young enterprising lad £60 or so to do it
maybe the huge increase in rape in the UK is encouraging them?
They do say it is very hard to get a conviction these days...
You would think bugs would slow a plane down, and maybe they do with some aerofoils. Rumour has it that some hi-perf (uncertified) designs are so critical they don't even like raindrops. But I've never seen any difference on my TB20, between clean and dirty.