Couldn't agree more elsLan. If a person is a smoker, you ask do they only smoke outside, do they pick up their cigarette butts etc. This isn't discriminating against smokers, merely doing appropriate due diligence on behalf of the owner, given likely scenarios that could come up.
BTW, there is nothing illegal (to my knowledge) about discriminating against smokers (or pet owners). You see it all the time in those flatmate ads looking for non-smokers. So discriminate away if you wish!
Unless the RTA says something about it? Can't remember seeing it but it wouldn't surprise me come to think of it...