If my outdoor thermometer, which I brought inside for a while, is accurate, I was at 49*F this morning. Still no heat on. We don't have central heat, just radiators, so I can't set a specific temp. If we could, I imagine it would be about 50 during the day, and off completely at night, no matter what (our lows rarely get below the high 20s).
When it gets cold, the air gets extremely dry, so I have no issues with mustiness whatsoever. When I shower, I crack the window in the bathroom (we don't have bathroom fans) and leave it that way for about 10 minutes after the shower, with the door into the rest of the house closed. Because it is part of our lease, I'm actually required to open up the house for about 10 minutes a day, no matter the weather. So I open the downstairs while I am upstairs doing my shower and dress routine, then open the upstairs (with the bathroom already have been opened) while I prepare breakfast or check email, then close the upstairs.