For me it depends on how difficult it is to get at said window. If I need to put the price up then I do so but most of the time it only adds 30 seconds to the time taken, and hopefully my price is good enough to swallow that without noticing.
I think an advantage with WFP is that on UPVC conservatories you can wash everything down and still be quicker than trad, so I wash these down, frames and all at no extra charge, the work is priced up exactly as if I was going to clean them the traditional way.
The only time I charge extra is if it going to make a difference in the time taken.
Velux lights I charge extra for though.
In the past I've found commercial customers the worst for asking for this or that to be done while we are 'at it'.
They always splutter then when you charge em' extra
I don't care though, they want the inside of a single door done it'll cost them at least a quid extra
Regards
Ian