| I like the back foot in a 45 degree (forward) position and I teach it in classes. Like any joint the foot/ankle has a range of motion. If the toe is ahead of the heel the final push at the end of the lunge (the main push coming from the quadraceps) has greater potential. The calf only adds a small amount to the lunge, but how many times were you just a "hair" short or late on an attack?
Since the push area of the foot is the ball of the foot, especially at the end of the lunge, there are basically two pivot points, the ball of the foot and the ankle. Like any joint, one that is already extended or partially extended, has no more, or diminished extension left. I also find the same to be true if you launch a flesche attack. The toe being ahead of the heel seems to make a lot of sense. Keeping your stance so that your heels approximately line up is so much easier with the back foot pointed forward slightly. Beginners that do not have years of the repetition in an enguarde position that seasoned fencers have, will have a much easier time keeping their knees and feet in order with the back toe pointed forward.
As far as balance is concered, I think that it is all in what you are used to. For instance, I started to do some PT on my left knee a few weeks ago and the lunge the physical therapist had me do was both feet facing forward and lunging keeping both feet forward. At first, I could not maintain my balance if my lunge was on my left leg (I am left-handed) but the right leg, that had no lunging experience to speak of, was easy to do. My balance was based on years of repetition doing it a certain way. After two weeks of doing the rehab the way the therapist wanted, the balance was fine.
Joe Biebel | 
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