Today I began reading A Dictionary of Modern American Usage by Bryan Garner, and learned this:

"Acronyms and Initialisms
An acronym is made from the first letters or parts of a compound term. It's usually read or spoken as a single word, not letter by letter (e.g. radar = radio detection and ranging). An initialism is also made from the first letters or parts of a compound term, but it's usually sounded letter by letter, not as one word (e.g., r.p.m. = revolutions per minute)"

To summarize: if you usually say all the letters (e.g MTC, IRS, GPA) then you are using an initialism, and if it is pronounced as a word (e.g. GAAP, FAFSA, NASA) then it is an acronym.

I am a little wary because I don't remember hearing anyone use the word initialism in normal conversation, (I'm fairly confident that acronym is generally understood to mean both things) but I'll be listening for it now.

I'm planning to eventually read this entire book, which could mean similar posts in the future.