Perhaps it would be useful, but I do wonder about its scope. The tag was created (by grovkin) with a description roughly synonymous with [US] federalism:
questions related to the balance of power between the US Federal government and State governments.
So it is basically an intersection of "federalism" with the US (tag), as it stands. I'm not saying this kind of intersection is necessarily a bad practice because we do have us-state-laws as a tag, for example:
Questions related to any laws enacted by any of the 50 states in the United States. Use the [united-states] tag alongside this.
What I am concerned about however is that Wikipedia has this narrower definition for "states' rights":
In American political discourse, states' rights are political powers held for the state governments rather than the federal government according to the United States Constitution, reflecting especially the enumerated powers of Congress and the Tenth Amendment. The enumerated powers that are listed in the Constitution include exclusive federal powers, as well as concurrent powers that are shared with the states, and all of those powers are contrasted with the reserved powers—also called states' rights—that only the states possess.
So with the description of the states-rights tag as created by grovkin, it may be broader than what seems to be the usual scope of the notion.
And as also reflected in some comments below divibisan's answer, Wikipedia says
Since the 1940s, the term "states' rights" has often been considered a loaded term because of its use in opposition to federally mandated racial desegregation and more recently, same-sex marriage.
I'm not sure we'd want "pro-life" or "pro-choice" as a tag, for example. In sticking with Wikipedia's [narrower] meaning of "states' rights", "reserved powers" would be a more neutral substitute, but arguably less well-known to the general public.
Anyway, in sticking with the semi-consensus from the comments to the other answer, I've added a synonym suggestion from states-rights to federalism. You can go and vote on it here if you think it's preferable to have it that way. (grovkin apparently did not have enough tag-rep to make that suggestion himself.)