Yes, best to not let it get to that point to use the point(you missed a great chance for a witty pun in English, there!). I have only used a knife once, swung in low with a hammer grip, and no problem, and it made a believer out of me. Your hand is going to slam the guard with a running start, the thumb will not help prevent it, will only get run over, and will slam guard anyhow. If we were talking larger sabre blades with their own mass, or even really large bowies (same thing), you can keep the thumb at guard, if no armor. But I will never remember any difference at the time and keep things simple.