Very interesting question.
I usually just say no. But if she's hot, it's the perfect excuse to try and engage her.
If I can see a guy near her, I'll say: "Nah, why don't you get one off your boyfriend."
But usually, if she's attractive, it's a great ice breaker.
If she bails straight away, she's wiped. Cigarettes in Australia are as dear as fvck ($22 for a 30-pack), but if it only costs me one cigarette to weed out another ho, then that's cool. She just gets the cold shoulder if she approaches me again.
I'll also offer her my unsmoked butt or offer her a drag of my already lit cigarette but rarely a whole cigarette.
Still, if you take a girl outside for a smoke, it's a good way to separate her from the herd and work your magic on her.