There are two each for lowercase and uppercase tags, and they are split in popularity. The hyphenated version being more popular for one and the non-hyphenated for the other:
[uppercase] × 13
[upper-case] × 36
[lowercase] × 30
[lower-case] × 19
The preferred tags should be [lowercase] and [uppercase], those being words often spelled without hyphens.
As someone studiously manually retagged all [uppercase] into [upper-case] (the opposite of what I proposed here, and only an hour after I posted this question, so I'd really be interested in your thoughts), it got me thinking: several questions are tagged both and every time either tag is appropriate it seems a single tag that represents both could be used instead. What could that tag be? [case-sensitive] seems appropriate and popular—moreso than all upper-/lowercase combined (just barely).
Even [case-insensitive] could be synonymed to [case-sensitive], since any insensitive question deals with case sensitivity issues. [case-sensitivity] might even be better than [case-sensitive], but it doesn't exist yet.
What is the single tag we can use to tag all of these similar questions? I vote for all the tags mentioned here to be merged into [case-sensitivity]. With one synonym voted-in already ([lower-case] to [lowercase]) and the multiple tag merges required, this needs a diamond's special touch in any ... case.