Is it an accepted standard? In my experience, no. Is it NEC compliant? Yes, provided the ground fault return path is 6ft or less and the fittings and flex you use are listed for grounding. Note that the 6ft. or less is for the entire run of the circuit. For example, you cannot run a piece of conduit with a 6ft flex whip into the panel on one end and a 6ft. whip to a fixture on the other end. That would be a fault return of 12ft. I think most here will agree either keep the run in EMT or pull an equipment grounding conductor.