I'd phone and ask them to change it if I were you. That's what they're there for, I'm sure it won't jeopardise your choice of accommodation. They will probably group all smokers together so you could be in a situation where its 5 smokers and you. Even if they smoke in their rooms, you're going to know about it, smoke gets everywhere.
I'm completely intolerant of smoke myself, it makes me choke and feel sick, so there is no way I could be anywhere near anyone that smokes! I think its all or nothing where smoking is concerned, with 5 in one house you're likely to be living in a continuous chimney. If all of your flatmates are smokers and its just you who isn't, I'd very much doubt they'd agree to not smoking in certain rooms or would go by the rules and not smoke in the communal areas.
You get a lot of arrogant smokers who have no consideration for non smokers, because, lets face it, they can't have thought a lot about it in the first place to be still doing that to their bodies. If you're living with smokers, you're probably very likely to be passive smoking. Thats just my take on it though!