Yes, questions about CR calculations should be homebrew-review questions; asking whether something is balanced is sufficiently similar to asking when something would be balanced
The current description of the homebrew-review tag is as follows:
For questions about evaluating the viability of new rules content homebrewed for an existing system. This tag should be used when locally-created, non-published content is at the heart of the question. [...]
Note that questions asking "what should be the CR of X monster" aren't about the viability of a new rule, they are asking for a determination of where to place something on the CR scale. Most of the homebrew-review questions include some form of the word "balance" (basically a synonym of viability) but here is a list of homebrew-review questions that do not contain "balance/balanced", and are neither closed nor duplicates. There are only 44 of these out of the 177 total homebrew-review questions which are neither closed nor duplicates. Here are some examples from the list put into some categories:
About spell level:
About item rarity:
About item pricing:
About monster CR:
It is pretty clear that many of these questions aren't about the viability of homebrew content, but instead are about where to place homebrew content on a particular scale such as CR, pricing, rarity, and spell level. Seeing as how these are 18/177 or about 10% of the question variety found under this tag, it is no insignificant number.
Though this is only the precedence thus far I do believe that "Where do I put this?" questions should be tagged with homebrew-review.
The skill sets required to answer such questions are sufficiently similar to those required to answer other homebrew-review questions
I believe this to be the case because they are often very similar to or even already a common part of the more obviously homebrew-review questions. For example, say a question is asking whether a spell is balanced, and it has a given level. A good answer does not simply say whether something is balanced or not, but will explain why and suggest ways to balance the spell, including changing the originally proposed level.
Somebody who provides good answers to questions about if something is balanced can also provide good answers to questions about when something would be balanced.*
The two type of questions ask effectively the same things
The usual homebrew-review answer is as follows:
This content is/isn't balanced, here are the reasons (and thus also suggestions for fixing it.)
And with these sorts of "Where do I put my homebrew content on scale X?" questions, an answer goes as follows:
This content is balanced if you put it here (basically, if you apply this fix)
I find that asking whether something is balanced is near-identical to asking when something would be balanced and so the tag is correctly used on both sorts of questions.
Though the tag description doesn't particularly suggest questions of the form "Where on this scale should I put this homebrew material?" are on-topic for the tag, I believe such questions correctly have this tag. The skill sets required are sufficiently similar to warrant doing so.
That said I'm unfortunately not sure how to update the tag description given that it has only 19 free characters left.