By cornering at low speed, do you mean slower than walking or jogging pace?
I'd say do whatever so long as you're in full control however riding with a leg out at slow speeds screams out of control to me.
At very low speeds (ie. circa walking pace), do not "lean off" the bike. The rider should stay more upright with feet on the pegs and the bike should be leaned over. One thing you give up when a foot is off the bike is the ability to steer with your knees at lower speeds. This way you can prop the bike back up with your bigger leg muscles if you have to suddenly come to a full stop and/or straighten out.
The above would apply to both on and off road at slow speeds.
BTW, I see this everyday with folks turning right or left at intersections. The worst is when the outside leg points skyward.