MLA convention for words and phrases used as words?
There doesn't appear to be a direct rule regarding these "words and phrases used as words". The closest I can find to an opinion on the matter is under an MLA resource for plagiarism, which implies that quotation marks are only appropriate when the words or phrases are directly borrowed from the original author or authors. Those would of course require a citation.

The article, to me, suggests that quotation marks shouldn't be used without a corresponding citation. It definitely leaves room for interpretation, however.

On a purely stylistic note, it is incredibly easy to start weighing a paper down with excessive quotation marks when you're new to a field because every other concept seems profound. The first draft of my thesis probably had these in nearly every paragraph - making it hard to read and just a little bit pretentious. So I would say it's probably better to err on the side of caution and limit your use of quotation marks for these types of words and phrases. You're going to explain the concept regardless!