In my experience there's very few "have to's" in pipe smoking, beyond the essential mechanics of stuff, light, puff - and even those are subject to endless discussion and preferences. To my mind, you smoke the bowl down as far as suits you at that particular moment or your habit. For example, I usually smoke a bowl throughout the day, lighting and then covering as opportunities come and go. But if I can, I like to sit and work through a bowl. In both cases I will reach somewhere near the bottom of the bowl, but how quickly and how far depend on circumstance and how that bowl is working out for me.
I've smoked to a fine white ash very occasionally over a very long time, but it's rare. It depends on so many factors - tobacco, pack, pipe, cadence, tamping, temperature, type of light used - that I really wouldn't think about it. It's certainly not a requirement. Also, given the number of bowls most people put through a pipe over the time they have it, I wouldn't worry about cake and damage. Providing you're not indulging in extreme piping and you give him some TLC as you go, your briar budddy will adapt to your particular - and possibly peculiar - style.
Andy
