I smoke to the bottom of the bowl 99% of the time. Around 85% of the time it ends in fine white ash, and I believe that there are 2 factors that contribute to this.
First of all, I always smoke half bowls, and sometimes less. I rarely have the time to kick back and enjoying a full bowl, and usually smoke once a day on the rooftop at the office, and even then I don't have more than half an hour tops.
Secondly I will almost only smoke vapers. Predominantly Anniversary Kake, AJ's vaper, FVF (sometimes spiced with a little 1—Q), Eggnog, Solani Virginia.
Even if there's humidity nearing the end, I usually finish before it becomes an issue and then wipe it out.
I find that I prefer smoking a half bowl, taking a break and smoking another later in the day rather than loading a full bowl and setting it aside midway.
Of all the brands I listed, FVF represents the 1% of the time where white ash is not achieved I usually end up with black ash, higher in humidity. I that case I dump it out as stubbornness results in a bitter flavor and why ruin a good experience. That and after FVF I'm usually buzzing on the nicotine and prefer to get on with my day.
If I pack a full bowl and want to see gray ash, I start out going as slowly as humanly possible, and then gradually go slower =)