Is case-by-case hyphenated?
Only when it's being used as an adjective. If one says "These
requests will be evaluated on a case-by-case basis," there should
be hyphenation.
However, if the phrase is used as a noun or an adverb, it will
not have hyphenation, like so: "We will review these requests case
by case."
Basically, if it's modifying, or affecting the meaning of, a
noun, it should be hyphenated. If it's affecting a verb (in the
second case, HOW reviewing is done), then it need not be
hyphenated.