I change the oil in my 4-cycle engines usually twice a season. Once during the middle of summer (around this time I guess) and once at the end of the season. Every now and again, I'll just do it once a season, but always after the last mow of the season, so that used oil doesn't sit in the crankcase and oxidize all winter.
I always keep the engines stored over the winter full of fuel. In my experience, even when using fuel stabilizer, trying to run them out of gas lets varnish build up. I keep the tanks full, and the fuel shutoff closed, and haven't had problems with startup in the spring. Our off season isn't all that long, though. Maybe November through March.