It is just slight, but it has definitely burned away a small bit of the front inner rim. I'm not too worried about it. It is a really old pipe, and it is turnout out to be really nice.
Is it preferable to leave a little bit of the discoloration of the stem, or better to take it all the way down to black? I'd rather not take off too much, if that would end up hurting the "value" of the pipe (I know it isn't worth too much, but I want to know the right way to do it).