As a personal preference, I'd love it if we didn't need the freeform at all. But looking at the reality of how the category tag is used, I often find myself frustrated - or hear the frustrations of others - about finding works that really are femslash between female-identified characters.

So I guess the answer depends on your fandom? If it's a fandom that's 95% femslash, you probably won't need the tag. If it's a fandom with lots of Rule 63 and background femslash, you'll probably want either "Femslash" or a subtag to make sure everyone can find your work without frustration. (Using subtags has the same effect as tagging with "Femslash", except even better, because your work appears in multiple filters.)