A lot of blends get bitter, sour, and/or hot near the end. If this happens, I just stop smoking, let it cool, and dump the ash and dottle out and load up a new bowl. Pipe tobacco is cheap, so it doesn't bother me.
I was under the Impression, that a pipe should always be smoked to the end to build up a protective ash layer in the bowl. It's supposed to protect the briar (?). This being said, I always suffered the last bit although I always hated it. I would be glad to hear I'm wrong.
PS:this is my first post on this forum. I smoked pipes for many years then I gave it up some 10 years ago. I restarted lately. Therefore many of my ideas about pipe smoking may have been proven wrong during my absence.
Stupid question: if I hadn't time for a whole bowl but only let's say half of one, should I rather just fill half of it to the bottom or fill it completely and smoke half of it?
When breaking in a pipe, I try to smoke the tobacco as far down in the bowl as possible. As others have said, though, excessive relights at the end generate a lot of heat and are not good for the pipe. When I am not breaking in a pipe, I smoke the tobacco only as long as it remains an enjoyable smoke.