We try to make sure that my brother doesnt watch 15 films even though he thinks that just because he is 16, he is able to do so.
We have a very tight household and even though a few swear words of the moment (like when you hurt yourself) is allowed, we dont bring profanities into general conversation.
Its the same with films, unless there is a film where you expect swearing, there shouldnt be a need for it.
It's guess work. Goes on genre/subgenre/actor/director/other. Slapstick comedy might have no swearing at all or with another actor and writer it might have it by the bucket load. You might have loads of 'gross out' moments. Similarly high school films might be fairly straight forward light coming of age dramas or they might be swearing/sex/drink drug fuelled films or something completely different. You have to guess.
The Sweeney I would expect swearing in. Other than that it has Ray Winstone in it and directed by Nick Love (and also written by him and Hodge) which might suggest that it's not going to be An American Tale.
On what you do at home that's upto you. As for the need for it, a higher percentage of swearing in films is in some kind of context than I think many people who think it should be virtually non-existent give the films credit for.
edit: There are plenty of films out there, with movie rental or download services I think it eliminates a lot of the problem. Yes, there is a social side of going to the cinema but that's often compromise if going with someone else. My favourite film is classified as a PG in the UK even though I don't have much of a problem watching 15 or 18s.