Is a powerback allowed by airlines as a safe maneuver?
Most modern commercial airliners are prohibited to power back.
Ground operations in aircraft fitted with high-bypass engines are usually restricted to idle and low-idle operations (enough to make the craft start moving, after which momentum enables further movement with idle only). 
Aircraft capable of power back are predominantly thus turboprops, several operators of these allow their crews to operate accordingly.

It's not in common use anymore, for the following reasons:


Thrust reversing can kick up a lot of debris, posing a safety hazard. This would generally preclude any ground crew or spotters walking under the jet as is often done during pushbacks.
The previous point's especially true for under-wing jet engine designs, so only jets with fuselage-mounted engines are allowed to do it, and most of the bigger designs (DC-9, MD-80, 727) are being phased out of most fleets.
Pilots in most craft can't see behind them, another possible safety hazard requiring multiple spotters to be in contact with the pilot.
Thrust reversing uses a lot of fuel, so it's usually cheaper for the airlines to use a guy on a tractor for an assisted pushback.
Thrust reversing is loud, and during a powerback that noise is in close proximity to the terminal. The tarmac's a noisy place, but usually not that loud.


If you want to see a powerback performed by a commercial airliner, fly out to L'Esperance in St. Martin. The runway there is very short and has no taxiway leading to the end of the runway from either end, so larger passenger turboprops that service that airport have to turn onto the runway about 2/3 down its length, face their desired takeoff direction, then powerback to the end of the runway in order to have enough to take off. The larger and (relatively) busier Princess Juliana International, on the Netherlands side, has taxiways and turnaround areas at both ends of its runway.