I'm just learning that smoking to the very depths of the bottom is not always pleasant. Typically, if the tobacco is too moist, the repeated relights into the slowly dwindling tobacco in the bowl make finishing the bowl more trouble than it is worth.
But there is a certain satisfaction in tapping out a bowl, having a fine rain of ash drift out, look inside and see absolutely no visible dottle.
I have never really gotten a mouthful of ash and i prefer to 'smoke it all'.
I've found that I really enjoy the bottom third of the bowl, especially when smoking flakes. I usually quit smoking when the dottle won't relight. And with that, I almost never end up with a bowl of the proverbial "fine grey ash" that you read about in many reviews. Quit when you think you should, and if that means dumping a little unburnt tobacco, so be it.
I smoke it tell it looses it's enjoyment. It is not worth it and takes away from the experience to force smoke all the way through. I have tried it a couple times and it just becomes more of a chore than enjoyment. I consider the last bit of tobacco a disposable filter.
I have found that there is a change on the flavor of the smoke when I get close to the end and that is where I stop...
I guess the difference on the flavor it's because of the change on moisture of the tobacco and the mix of some of the ashes from the top of the chamber going down with the tobacco leftovers.
I usually remove ash at about 3/4s done. I give the bowl a good tap and invert and it falls out.
I then insert a pipe tool to find out whether what's left is worth smoking, if there is enough tobacco left such that smoking it won't be too hot. When it is insufficient, less than a quarter inch, I dump it.
About the only time I get an ashy draw is with my cobs - I ditch the filter, so the draft hole is wide open, and I will occasionally get a bit of ash at the end of a bowl. As I've learned to smoke more slowly, it's become rarer and rarer that this happens - one more reason to slow down!