Ian,
Personally, I would question it, I just can't see how an extra charge can be made to do the same job, 1st cleans maybe, but regular cleans I couldn't justify.
You say it takes extra work, a minute or 2, we know how quick wfp is.
Bird mess can become rock solid very quickly, so it doesn't matter if its 4,8 or 12 weekly its still be solid.
Extra paper work, a tick, cross or update some sort of software, what ever way we keep our records, you still got to do it every time ,4,8 or 12 weekly, its still got to be done, so not sure why thats extra.
Lets face it what we do for a living is not rocket science or brain surgery, and theres lots of us out there waiting to jump in, I play safe and keep the customers happy.
darren