As has been stated, the ash acts insulation preventing the cherry from getting very hot and the smoke turning bitter. It is wise to leave the ash on for a while. I used to let it stay on until it fell off naturally, but after too much ash on my shirt and in my lap/drink/etc I have found that I ash just before it will fall on its own. This is at a different point from cigar to cigar, so no general rule like 1" ash works. Sometimes it is 1", others at a half inch, others at 2". Construction of the smoke and smoking conditions like wind all play a factor in determining when is the right time to ash. As with everything - experiment and see what you like best
