Races lost to glides far outnumber races lost to an extra stroke. You slow down when you glide.
Gliding in to a finish is a no-no in swimming. Kids are taught that from the very beginning of their competitive swimming and they often ignore it,.... and lose the race.
If one is close enough to the wall that one can stretch to hit the button or will be able to do so within 100ms, I would think stretching would save time. That having been said, the cost of taking an extra stroke when not needed is below the cost of failing to take one when needed. I don't think swimmers can really judge each other's positions effectively (I wonder if it would be useful to include cylindrical convex mirrors on the bottom of each lane to allow swimmers to see what's going on in all the other lanes) but if a swimmer knew another swimmer was about to touch the wall before he could finish his stroke it would not seem reasonable to put his arms immediately forward and hope he's in range; if he is, he wins. If not, he would have lost anyway. Of course, if the other swimmer wouldn't have touched the wall before the stroke was completed, and the lunging swimmer wasn't in range but would have been after completing the stroke, then the lunge would lose the race unnecessarily.
I don't doubt that it's better to train people to keep stroking until they hit the wall, than to train them to try to judge end end, but I would think that expert swimmers could sometimes benefit by the latter strategy.
BTW, totally different subject: one thing I’ve thought might be an interesting event would be a 100m individual timed run, in which the runner could place the starting blocks 3-50 meters before the start line. The runner would be told to start at leisure, and would be scored on the interval between (1) completely clearing the start line and (2) completely clearing the finish line. The contest would thus measure the fastest average speed that could be maintained for 100+ meters, without regard for starting reflexes or initial acceleration. Requiring the runner to completely clear the finish line would avoid any advantage that might be obtained via diving or other such techniques; measuring the time between clearing the start and clearing the finish would make it a true measure of the time to travel 100 meters.