With regards to Twilight I read the books then previewed the movies before letting my DD watch them and I will not let her read the books yet as I feel they are a lot more in depth & explicit than the movies.
Her & I often have movie nights & I'm comfortable letting her watch M movies as long as she's with me, she's not to watch them alone as I will turn the movie off if it's to explicit. Any thing a bit adult in the movie is discussed either at the time or used as an opener for a conversation down the track. M15 movies are a no no as there's more drug use, foul language & overly sexual themes. But a lot of old M15 movies would now come under the current M banner so I'm happy for her to watch those ones.
If she has friends staying over I always check with the other parent as to what they feel is appropriate to watch as everyone is different.