Okay, as I type this, I think I actually came up with two questions:

1) Should I avoid creating tags that would only be used once? I assume the answer is yes. When a tag is used only once, the number 1 is highlighted red and warns you to populate the tag when you mouse over the red 1. However, I still see odd tags used as jokes which seem to stick around, though, probably because it's funny that such a rare thing exists.

The answer to this may help give context to question 2.

2) If a tag is used a few times, like single digits but more than once, it's allowed, right? I've seen edit wars removing the same rare tag multiple times, even though the tag is most certainly applicable. If the answer to question 1) was yes, then it's possible there was a time the tag had 1 use, but now it has more, so I think the edit war should quietly end with using the tag. The fact that the "red 1" warning asks you to populate the tag suggests rare tags should be used more, not less.

(I was hoping there'd be a general answer, instead of having to ask case-by-case for something I know will be extremely weird and kinky. I mean, it's niche by definition.)

It should not be a big deal as long as you exercise some restraint. Look at all the unos:

https://e621.net/tags

sentient_breasts

It doesn't seem a big deal if it actually applies, but some tags are marked invalid for a reason. Removing an applicable tag instead of changing it to one more appropriate (say, 20-presence tag that every single case meant same thing as 15K-presence tag) is considered abuse, actually? The red warning is to remind you that maybe you made a typo or should really, really consider if that tag already exists as some variant, or that it's invalid. I've been adding some related but not quite identical tags to wiki like for utility_belt and bandolier. Seems that people mistake one for other sometimes. If you type out a tag partially, and right-click on the resulting suggestions, you can open that specific tag search in another tab, and then click on the "?" mark to go directly to Wiki article.

BTW: I've been dealing with some tags that (at least for foreseeable future) have low presence but are applicable to specific fetishes. I'm still only halfway through one of my search results using arrow button to review all. I'll get it done eventually, I hope. XD

no those tags belong in a museum

These tags aren't really rare. They're just very low tagged. All tags must start at 1, but some rapidly take off into the thousands while others apply for quite a long time to one post only, just waiting for a second tagging to happen. Of these one-post tags, many are indeed goofs to be removed. Others are unnecessary because other, more established tags do the job already. Still, there's plenty that are valid but only apply to maybe one or two posts at the moment simply due to a lack of material uploaded here for the time being. Chile's Navy Day might currently be on only one post, but it's just as relevant a tag as the Fourth of July and could conceivably be tagged on another post someday. We just have to take these tags on a case-by-case basis as we do all tags.

in general it's fine to use tags that have low usage. although, if the tag you're looking at seems way more underpopulated than you expected there might be a more well-used tag that functions the same way, and you might want to ask about it the ''do we have a tag for that'' thread thread.

If the tag is applicable and has no better tag then please use it, that's how we get less common tags off the ground. Don't go crazy though.