Please bear with me while I climb on my soapbox and rant for a minute or two.
On occasion, while enjoying a cigar in mixed company, my nostrils have suffered the slings and arrows of outrageous odor. This is a direct result of stubbing out of a cigar like a cigarette, a process that transforms the intoxicating aroma of even the finest cigar into a malodorous, offensive affair. That stench is capable of turning-off even the most die-hard aficionado. It's also terrible form.
An officemate of mine (who really ought to know better, mind you) has taken to the habit. When he finishes smoking, he dutifully grinds it down into the ashtray before walking out of the room, thus escaping the foul stench he has just created. I've since brought the matter to his attention, and I'll admit he has seen the error of his ways.
Good form dictates than when one is finished with his cigar, he lays it down in the ashtray to die a dignified death. After all, hundreds of hands have worked together to produce your beloved vitola; it is a product born of the earth and enjoyed in a moment of leisure and relaxation. Surely it deserves a kinder fate than that of a cigarette.
But perhaps more important is that the gases and tars contained within a cigar are mashed up and expelled when ground out. Cigars that are stubbed out aren't going to go out any faster than those that are left to go out by themselves, so unless you're doing it for dramatic effect, leave the stubbing out to cigarette smokers.