In regards to what gets tagged with it, it's a problematic tag to tag things with 2 different reasons.
1 is Spoilers. If a work has a upsetting end in which she cheats, or leaves her for a guy, etc, then that dramatic twist is removed by way of the tag. And with no spoiler tags, theres no real way to avoid this other than not tagging it. After all, these stories weren't originally printed with a warning at the start saying "Warning, she cheats at the end", so there's no real reason to warn people.
2 is the problem of it being a Yuri Manga. What exactly would you tag as bisexual in a yuri manga? There's a number of things in yuri manga that could be described as bisexual, but in the context of yuri isn't exactly that. Theres the "I usually date guys, but I love you regardless of gender", there's "I've dated a lot of guys, but we broke up after one week, and now I'm with you I realise why", even the dreaded "It's just a school girl phase". Would these be tagged? Or would only some of them? It's a difficult situation.
Surprised you didn't hear about them, they are quite infamous among some people.
But then again i too came to know about bi just two years ago.
In real life, most of people avoid bisexuals cuz of them able to fall for both girls and boys. And high chance of cheating.
In short, you have to keep on guard with them with both parties. No resting time.
FFS. Stop with this shit again Ulq.