I understand what you mean by wanting quality customers over the quantity - but I still believe in a lot of cases if you force customers into terms they may be more likely to cancel or be unhappy with the service.
To be honest if a salesman came to my door selling doors, but told me i could only have a blue one i'd probably be pi**ed off. in the same manner if i was a customer and a windy canvassed me, but when i say i would only want it every 2 months, and he said no sorry i only do it monthly i would probably tell him where he could shove his wfp.
i guess in some areas one technique would work better than others...and you have to play it by ear...