As far as I know, you'd have to use one of the location profiles like GPS or cell tower to do that if it's based solely on leaving a location.
You might consider something else though. Do you use Tasker to start any other programs or anything when you normally leave your house? Before Slacker was updated a couple of months ago, Slacker would always hang up on me if I started the program in the house where I had WiFi and then took it on the road out of WiFi range. Since I only listen to Slacker on the android in the car as a rule, I set up a profile to disable WiFi whenever the Slacker app loaded. Worked like a charm. So if there's something else you routinely have Tasker make your phone do when you leave home, you could also piggyback shutting off WiFi with that.