I've been giving out Christmas cards since yesterday, and aim to get them all out to my custies by the end of next week as I clean their windows. The rest will be posted.
I think it's pretty acceptable to receive a Christmas card from any company you do business with on a regular basis.
I'd be pretty shocked if any of my customers were offended by me giving them a Christmas card. The only thing really is to consider is whether or not your customer celebrates Christmas. If your unsure send them a 'Seasons Greetings' card instead.
I've addressed my cards to the person I mainly deal with. If I've built a good rapport with the custy, i'll address it as To John and family, however, I've played it safe with a few eg To Mr and Mrs Smith.
At the end of the day, businesses send Christmas cards to advertise, and thank customers for their business.