I've seen a couple of videos of P-51s with gear problems being landed wheels up, both had the engine running. I imagine it's far safer to have power available to go around if necessary than suddenly find yourself trying to stretch a glide...
On the other hand I seem to recall that when Paul Day had to land Maurice Bayliss's Spitfire Tr.IX wheels up at Coventry a few years ago, he stopped the engine before touch down. But then again he must have hundreds of hours logged on Spitfires.