Answer by Alan Chestnutt:
P-lip stems do not help if you have a sensitive palate. You will just end up burning the roof of your mouth badly instead of your tongue. Especially as the P-lip stem emits the smoke in a concentrated narrow stream.
I have found the opposite to be true from the first answer. If the tobacco is too moist then it leads to 2 problems. Firstly it is harder to keep lit and will most likely mean you will instinctively draw harder on the pipe to keep a good burn. This can heat the tobacco too much. The moisture in the tobacco can then form steam which can bite the tongue with a steam burn.
The second reason for tongue bite is a chemical reaction with a constituent in the tobacco. Some people get tongue bite from red virginia tobacco and others from aromatic tobaccos. Personally, aromatic tobaccos used to burn my tongue and I believe it was the flavourings used that created a chemical reaction in my mouth. I have never had a problem since moving to natural tobaccos.