The brake bands should not "stretch" per say. However the brake material will wear. Even this should happen at a slow rate if they are adjusted properly, because the brakes are only used for parking. Make sure they are adjusted correctly, checking both clearance at the brake lever to the frame(apx. 2") and at the brake rod to actuator(located at the front of the brake assembly) you should have 1/8" clearance between the actuator and the lower nut. If this clearance is not correct you will wear the brake band out faster. As always check your owners manual for proper brake adjustment (page 20 on current models. If you experience further brake problems check with your local dealer, as this is a liability problem.
Hope this helps.
P.S. i have seen some replacement brake bands that are out of shape and will constantly rub no matter what adjustment you preform.