I find the need for ashing a bowl dependent upon a few forefront factors: tobacco cut, packing technique/finesse, tobacco moisture (when packed to be smoked), and the casing/topping applied.
Flakes and coins almost always seem to need an ashing if I don't rub them out enough before packing. Deeper chambered pipes can facilitate the need for an ashing moreso than shallower chambers, especially so if you are a wet smoker, or the tobacco is otherwise too wet (gurgling and the need for excessive relights during the last ~third of your bowl). If you are excessively relighting from the get-go, try drying the tobacco out a bit more before packing, pack lighter, and/or get an even initial charring light and ember going. Remember to tamp lightly. These will mitigate the "need" for ashing, though may not eliminate it. The internal engineering of the pipe will also be a huge factor on the burn of the tobacco. Is your pipe broken in? Couple all of this word-salad in with your smoking prowess (i.e. cadence, clenching, technique, etc.) and I'd say there's an amalgam of possibility in the age-old question of to ash, or not to ash, or rather, the need to (or to not).
I try to kick the ash, rather than getting my ash kicked whilst smoking. Some great advice that I can (mostly?) remember goes something like this: dry longer than you think, pack (and tamp) lighter than you think, smoke slower than you think, and you'll enjoy more than you think. This should aid in attaining your ashless smoking experience. Sometimes a pipe just abides, man.