One of the things I train is that I would rather do a decent job of making a poorer close forced landing site, than a poor job of reaching a better one. If you have any need to stretch a glide, that means that prior to the "event" you chose to fly over territory which was lees suitable for a forced landing. Sure, we have all done it, when our confidence about the airplane was good. And, I have certainly remained over suitable areas the whole time when I was not confident about the engine! Remember, if you have to actually force land power off, you'd rather misjudge and go off the far end at 20 MPH, than misjudge, and crash short at 65 MPH! I want my students to choose a good enough close spot, and make a good job of getting into it. So, for me, "best glide" will be a brief period, usually right after the failure, while you diagnose, and choose your [nearby] spot. After that, though we won't waste a glide, I'm happy to see an approach set up which includes a lesser flap setting and a sideslip to get it. You can always undo a slidslip to recover more glide, you cannot retract flaps to do it! So, for me, any flap use for a power off forced approach will be very late on the approach, but then, as much as possible to shorten and slow my ground roll.
I have had four power loss forced landings over the years. I have been very lucky to have never damaged a plane doing it. But, I have been right up to the far fence a couple of times!