Changes in temp as it sits can allow condensation to contaminate the oil. I change mine at the beginning of every season (put less than 1,000 miles a season on it). When I dust things off in the spring, the dipstick is usually frozen in the tube with rust. I start it monthly, too, and let it get up to operating temp. If I'm feeling ambitious I'll take it around the block but I'll at least run the trans through the gears while it's running.
Cliff Notes: Change the oil. It's cheap insurance.