When is it safe to stretch out my face and floss teeth after a fat transfer?

Gentile washing and teeth brushing and flossing is fine. But to repeat the answer to a similar question: Fat transfer, unlike inert synthetic fillers, are micro grafts of living tissue. The long term survival of the living grafts is dependent on ingrowth of new blood vessels. New blood vessels are very fragile. Frankly, I'd tell you to wait 12 weeks before applying any shearing forces to your skin. 

No one know how long it takes for the transplanted fat to stabilize in its new location in the face but it probably will either survive and thrive or die and disappear by the two week mark. For this reason we ask our patients to be very gentle with their face in the first two weeks after surgery (no rubbing, no sleeping on your face, no heavy manipulation). After two weeks you should still be gentle with the face (gentle cleansing, minimize rubbing, no sleeping on your face) but I don't think flossing the teeth would be a problem. That said following your surgeons recommendation is always the best advice.