I used to 'bite' at the 'not this month brigade', but now have the following policy:
I have my terms and conditions on my chit, and every customer I have should know that I clean their property once per calandar month; unless we've arranged otherwise.
I will drop any customer who asks me not to clean their property when I turn up.
If I'm given warning, such as, 'we're off for a three-month round-the-world holiday; would you give us a miss for a couple of months', or 'We're having building work done'; then yes, I'll miss them for the given period.
I think you've just got to use some common-sense. If you suspect they're extracting the urine out of you; to save themselves some pennies; drop them.
If they have a genuine reason and have the decency to warn you in advance; then no problem.