I
never knock when I arrive to do a job. When I quote, I offer the option of a phone call the night before - in case of locked gates etc. and if they say they don't need one I say that I'll just "turn up and do it". That sets the terms of the agreement.
Knocking before you start is an invitation for them to say "not this time".
Having said that I've had the odd few over the years stop me halfway through the job. When I was trad I made sure I soaped all the windows before I stopped

Not so easy to register your protest with WFP.
Cheers,
Ian