You should send feedback when a story strikes a chord with you. That is, when the story makes you smile, laugh, or cry.
You should send feedback when you want a sequel to a story.
You should send feedback when there was something in the story that makes you want to read it again.
You should send feedback when you re-read a story for the fifth time, because you like it that much. And be sure to write them again the twentieth time. : )
Do not send feedback when you are angry at the author for writing the story, cool off, go read something you like - a day or two later if you feel you have to - write a well-reasoned note as to what offended you, without any personal attacks. I would hope that printing out the letter, and burning it would clear your desire to send feedback to the author - but if not....
Do not send feedback if you accidentally read a slash or het story. If the story was not marked on this archive - contact me.
Do not send feedback to slash authors if you are just going to tell them not to write slash. This goes for any other subgroup of fanfic, if you don't like it - avoid it.
Do not send feedback if you have more negative points about the story than positive ones.
Feedback need not be lengthy. Feedback should be constructive. Feedback should never include put-downs about the author.
When feedback is positive, it can be short. Smiley faces are my favorite type of feedback to send and receive. It's quick, conveys the point, and is better than nothing.
When you want to be more detailed, and send constructive criticism, always start and end with positive notes. If there are several issues with the grammar of a story you read, and you wish to tell the author how much you loved her story, make sure that the positive outweighs the negative.
Do not beta a story. Feedback really should never be more than a page long. It is not your place to edit that person's story and send it to the author. The story has been archived or published, and submitted for the reading enjoyment of fellow fans. If the author wanted a beta, he/she would have already gotten one.