The question for me is: Do I add Tags based on every kink that appears in the story, or do I only Tag what is an actual plotpoint?
Like, if your MC were to overhear a conversation between two girls, talking about how much fun they had with each other last night and going into great detail... would you Tag the story with "lesbian" because of that, or would you only Tag it if the MC actually witnessed/participated in it?
Tags are for the benefit of the reader, either to find stories with specific tags or to avoid them. So you have to look at it from that perspective. Is your example descriptive? How would a reader feel if he wants to avoid lesbian scenes and encounters that paragraph? Is it a main part of the story?
As you can see it's pretty arbitrary and no two authors tag stories in the same way. One of the most important reasons for tags is too help avoid squiks, so take that into account when you add tags.
There are other threads around here about this, but I think it boiled down to how the author wants to handle it.
For me, I think of codes as being in different categories. Some are pretty innocuous - M/F, Romance, Science Fiction, etc… Some have a high squick factor in that there appear to be a lot of readers who are turned off by them. Examples might be M/M, Rape, BDSM (and its variants), Incest, Snuff and Necrophilia.
For the first class I mostly tag for search, for the second I'd tag for squicks. So, in my most recent story I coded it for Rape, even though the rape isn't very detailed and only takes up one paragraph.
As Tendertouch, errr, touched upon, this has been asked many times and you may wish to search for it in the forums and read the posts.
The general consensus is normally to tag for the main themes of the story, and not bother with incidental actions, ie two female characters kissing passionately and no further, would not warrant a FF/lesb tag as those reading your story for the FF content would be angry that it was only one kiss and nothing else. Likewise, if a female character is slapped once, then no need to tag it for violence unless she gets beaten half to death in the next chapter and you go into it in great detail.