I have 3 cars, they all sit on the tires year around. One, a 1970 Chevelle only gets a few thousand miles a year on it so it sits most of the time. Flat spots, yes it happens but usually works out of the tires after a few miles. Leaving the jacks down? Assuming hydraulic jacks the pistons are exposed to the elements. The hydraulic lines are also under constant pressure. Supposedly they're designed for that but your tires and chassis springs are also designed to support the weight of the vehicle all the time. I leave mine on the tires and jacks up 'most' of the time. I say most because the jacks are down when we're using it or if I need to put the slides out to clean or work inside it.
Oh, according to some tire experts on the forums you should replace your tires every 3 years because somebody had a blowout on a 4 year old tire. I hope nobody reports a blowout on a 2 year old tire or we'll have to replace our tires every year.
