Well, figured I'd add my 2 cents ...
I see trouble no matter which way you go. If you take money, they will expect service. If you don't plow as soon as they think it shold be done, or as good as they would like it done you have a problem.
If you discuss your costs and expectations, that can also p*** em off.
I'd not discuss it and surely not plow their's if I had the flu. I'd do it when / if I feel like it. If that is not good enough for them, they'll look for another solution, but then they'd be coming to you, or hiring a service. Or maybe they have one of those huge SUV's and just plan on driving thru whatever is not plowed.
I do a few neighbors drives on occasion. Depends on how much snow and how much time I have. One Thanks every time. One thanks and offered coffee. One never said a word, but only their college age daughter was there that winter and she is a big introvert. I like not having to do any extra drives ....but if I want the seat time and I'm not freezing my behind off, I can be neighborly. If you get into any business type agreement with them, then neighborly goes out the window and you'll have expectations on you. If it was me, I'd try to avoid that situation.