Always as it's courtesy but I would not ask an open ended question which as you say does give them an opportunity to possibly decline, I usually just say hello and have usually dragged half my hose out and have brush in my hand, I have had the odd one who does say "is it ok not to have them cleaned this month" I then say sorry I can't as you have not given me enough notice to fill the time slot with another customer.
Graham
time out over a month how much time you waste doing this, you will be surprised I used to do this but I stopped for the following reasons:
Wasted time standing there when could have started and finish quicker
customers sometimes said can you leave it this time
stand there waiting and no answer (wasting time)
they are three that comes straight to mind which is enough not to knock first.
Now we start then knock, still curtious and cuts down on wasted time, if customer comes out and says not this time (which is very rare) we offer to clean the front only at min charge as we have already started, they normally just say oh do them all if not at least we have not cost time and money.