I smoke - to legendary status, but my characters only smoke if I really think they have that character trait.
However, it can be useful in signifying certain things or simply rounding out a character.
Examples... In one of my stories a model is being photgraphed and throwing a strop. She storms off for a cigarette. A hell of a lot of models smoke and it also gave her the excuse she needed to storm out.
A grandfather gets up from a table and says he is going outside to smoke his pipe and asks his grandson, recently back from war, if he would like to join him. Pipe smoking appears to be (appears) a leisurely, gentle pursuit, which the man embodies. Secondly, this is his opportunity to talk to his grandson alone.
After witnessing a suicide, a man, in conversation mentions that he has given up smoking. A scene or two later, he is lighting up. To me, it showed a common reaction to stress. I didn't make a big deal of it, just stated it.
In another, a character has the habit of flicking away a finished cigarette between his thumb and second finger. In the novel he does it three times. It's like a little character quirk.
I would say that about a quarter of my characters, maybe less, smoke, which is pretty much in line with most statistics in most countries. I do know which ones would not, though, and would not make them smokers.
I think cigarettes are one of those useful little props that we can use, much like glasses, a drink, fiddling with clothing or hair, beards etc. Nothing more.