This is a combination of rant and ramble.
I don't really hold with the process of giving each piece of my writing a rating like a film. It strikes me as inappropriate, because while film ratings are based on film images, and on what you see, the images you see inside your head when you read something I've written are entirely up to you. I don't go into great detail about what the characters are wearing, so if you want to imagine them naked, I believe you're within your rights to go right ahead. It's not as though I could do anything to stop you.
However, I've run across a lot of fandoms where the convention is to put rating information onto each story, so that those who don't want to read about icky stuff (sex, relationships, heterosexual sex or relationships, slash, etc etc etc) will be able to avoid that which they'd rather. This is also fair enough, since "don't like, don't read" is only really workable if you've enough information to make the decision on in the first place. So I tend to put a rating on the top of each fic, purely for the comfort of such readers.
A word of warning, though. I don't use the US system of ratings. I was born in Australia and I have lived all my life in Australia. The Australian system I understand, so it's the Australian system I use. You can find out more information about this particular system through the Office of Film and Literature Classification.