From “Ask Kit!”:
Q: Do I allow guests to use the laundry facilities?
A: There’s no right or wrong here. It’s what you feel comfortable doing and where your laundry facility is located. You probably wouldn’t want guests in your private quarters doing laundry but wouldn’t mind them using public space to do their laundry.
I’ve seen a few B&Bs offer that as an amenity and others offer it as an income source. Location matters some in your thinking; a remote location might need guest laundry facilities more than an urban inn. Most B&Bs I’ve been in don’t offer on-property laundry service though, so follow your heart and business style.