So, virtually every tobacco advertisement and review contains the line "Smoked down to a fine white ash". Its almost laughable when I read it...its the excepted and required statement.
This constant presence has seemed to drive the feeling that you're "smoking wrong" unless you smoke until you look into your bowl and find there's nothing left and you've consumed the last molecule.
However, many of the experienced smokers here will agree on a general concept: Do what works for you, and stop worrying about everyone else. How to pack, when to tamp, dump the ash, smoke to the bottom or not, yadda yadda....everyone eventually finds their stride.
I stop smoking when I can tell I'm going to have tongue burn or there's just no more flavor. As cigrmaster mentioned, no point in smoking something you don't enjoy anymore. And you're throwing away probably 10 cents or less.
When do I hit that point? Varies all the time. Sometimes, I smoke to the bottom. Sometimes, I toss half the bowl. Usually, its somewhere in the middle and I end up tossing maybe the bottom fourth or fifth.
Enjoy the tobacco and pipe, and worry less about being perfect. Paradoxically, the less you worry about it, the more you'll probably smoke all the way through.