I don't have a problem with one offs. In fact, I find that if you take them on, as I do, they'll phone occasionally for repeat cleans, so they aren't really one off cleans at all. I have plenty of these on my books, and when they come in, I fit them around my regulars and they give my income a welcome boost.
One could look at them in a similar way as add-ons. I mean, who has their conny roof or gutters cleaned every month? And why turn away work if it makes you money?
John