Each airplane will have its own approach speed and stall speed.
Fly the airplane as outlined in the operating handbook and be accurate in your flying and you will be safe.
Using power after the round out only extends the time it stays in the air.
If the attitude for that airplane is correct for the touch down and the rate of descent is correct the landing will be safe....as long as the thing is not drifting sideways.
Each airplane has its own touch down attitude, think of the space shuttle compared to a light airplane.
P.S. the space shuttle pilots did not use power after the round out.