My experience with smokers and tenants in general is that they will get away with whatever they can. If you let somebody slide once, it will happen again.
I don't insist that tenants are non-smokers, because there's no way to control what they do outside, and they'll tell you they don't smoke if they think it will result in not getting the rental. But my move-in sheet has a checklist so tenants can report anything wrong with the property, and there's a space to check 'smell of smoke', 'smell of pets', and 'any other smell'.
However, I will admit to having a personal prejudice against smokers. I just can't conceive of how somebody could knowingly willingly choose to blow their hard-earned money killing themselves, or could let a drug control them. To me it shows a complete lack of self respect, and somebody who doesn't respect himself won't respect anybody else either. While discrimination against a particular race, gender, or other quality that is neither better or worse than any other is wrong, discrimination based on a choice of actions in not wrong.