You get paid to keep the glass clean, if its still clean from last time job done
I agree with tosh
I wouldn't take the mick . But if I walked into an empty office and the glass was still spotless I would probably run a cloth over them quick at most.
I bet most who do lots of commercial would do this , tosh isn't recommending that you con them but that there are ways to be quicker , internals aren't the same as external , it's only high traffic areas like doors etc that get dirty month to month