You need to scoop out the ash as you smoke, unless you like the old ashtray taste when you get towards the bottom, and like to re-light often. That's why pipe tools have the little scoop on them. Check your bowl every so often while you smoke. If you can't see any burning embers when you puff, it's time to scoop out the ash. Dumping is not recommended because oftentimes, the whole tobacco load will come out, and that will be the end of your smoke.
I can almost always smoke all the way to the bottom of the bowl with no ashy taste, no relights, and nothing left but a fine, grey ash...no dottle at all.
The only exception to this is if I accidentally get ahold of some cheap tobacco that just makes a gooey, tarry mess in the bottom of the bowl. Wet tobacco does not burn. It's caused by additives and heavy casing, which I abhor. At the first hint of any gurgle, I dump the pipe immediately and get rid of the tobacco. I won't smoke that blend again.