I'm by no means an expert but PA39 summed up my point of view beautifully.
Rule 1. NEVER extend full flap until you are assured of a landing ! Pretty basic training......that doesn't seem to be taught anymore.
I was taught to have full flap out by about 400 feet, but it wasn't until my first Chief pilot pointed it out that i realized that this practice is "unsafe" in an engine failure situation. He suggested putting out full flap going over the fence, however I feel that's getting a bit late. I tend to throw out full flap when I'm assured I'll make it to the clearway, it leaves me enough time to stabilize the aircraft before the flare and if the donk did go U/S I'd be happy to put it in the undershoot if needed.