Is it better to start a car that is sitting for a while in cold weather
(it's 12 degrees now) or wait until it's in the upper 20s? Should I
idle it once a week - and when?

Sorry for the offtopic, but you guys are around cars all the time and
would know this . I have 3 cars, one daily driver, the others under car
covers. How do I keep them ok in this subfreezing weather?