I've been running direct the whole time i've had my bike, and at 30+mph with a good amount of chain wobble; no problems. I've had more problems with it being too tight than too loose. my rear tire has dropouts that go straight back, so I can adjust slack at the wheel instead of how the motor sits. no rub on either the frame or the tire. and thats with a 2.125" tire.
DITCH THE IdLER