I agree with Mikey, having garage-stored countless cars through long Michigan winters over the last 40 years, and his winters are a lot colder than mine.
It is NOT a good idea to "put the car up on jackstands"; I know Dick Guldstrand, and I'm sure his shop's recommendation is based on the cars they create, service and modify, which use race-type suspension bushings (urethane, Delrin, bronze, Heim joints, etc.), not OEM rubber bushings. Most people don't understand how OEM rubber bushings work; the rubber is bonded to the inner sleeve and to the outer sleeve, both of which are locked solid in position (the inner sleeve to the frame by the through-bolt, and the outer by being pressed into the control arm); there is no relative motion between either sleeve and the rubber - all motion takes place within the rubber itself by twisting the rubber in torsion, and the bushings are torqued at normal ride height so there is no torsional stress at all in the rubber at design height. If you put the car up in the air and let the suspension hang in full rebound, the bushings are seriously over-stressed in torsion and will eventually either deteriorate prematurely or the bond between the rubber and the sleeves will fail, destroying the bushing. "Flat Spots" are highly over-rated, and are mostly an "old wives' tale", especially with radial tires; any that occur disappear after a few miles of driving.
It should also be noted that the GM Powertrain engineer's preference to put oil in the cylinders and start the engine occasionally is based on storage for periods in excess of nine months; for normal 4-6 month winter storage, that's unnecessary. If you're going to start it, drive it at least ten miles so the OIL is heated to normalized operating temperature; just running it until the thermostat opens DOES NOT heat the oil up hot enough to boil off the condensation that forms in the oil pan and the rich-mixture blow-by contaminants that form every time you start it from cold; that just sits in the pan for the rest of the storage period and makes the oil more acidic. Just change the oil and filter, drive it a few miles, then park it and leave it alone. Seals DO NOT "dry out" during 4-6 months of storage - that's another "old wives' tale".
