I am completely comfortable with DP to ask him anything. The answer isn't always yes but if he smoked of course I'd ask him to stop. Him getting sick and/or dying is my worst nightmare. There is a reason married men live longer than single and part of it is because their wives help with their health issues. I nag him to go to the GP and I am bloody glad I do!!!
If it was a new relationship and he was already a smoker I certainly wouldn't 'tell' him to not smoke but I would certainly let him know I would be totally supportive if he wanted to quit.
A friend of mine lost his father to cancer as a result of smoking and you bet your bottom dollar he tells his wife her smoking is a deal breaker.