To be technically correct, it is undesirable and unnecessary for the flame to ever touch the tobacco. In most pipes, you can light the tobacco as it burns down by holding the flame slightly above the rim.
I hear people talking about not smoking the dottle. Most of my bowls are smoked to ash. Probably helps that I dry my tobacco adequately. I don't see any reason the bottom of the briar needs any more protection than the sides/walls. Someone help me make sense of that. To be clear, if you are holding your flame inside the bowl and puffing like a madman in order to smoke those last few strands, quit. As previously stated, you risk damaging the bowl, you're adding to your tongue burn problem and I can't imagine it would taste very good. If mine burns to the bottom with a reasonable amount of relights along the way, great. If I empty the bowl and there is some dottle, equally great. The only thing that matters is that I enjoyed that bowl of tobacco.