Do I use an apostrophe if I'm referring to several medical IVs? [duplicate]

The Chicago Manual of Style has an interesting way to address this: they omit the apostrophe, unless there are periods in the abbreviation. So this would give you ATMs, or alternately A.T.M.'s. (A.T.M.s looks weird.) chicagomanualofstyle.org, "Plurals"

This page indicates that acronyms ending in the letter "S" get an apostrophe, something I've seen before, but can't find in a general reference. So one would write ATMs and SOS's.

A page on the North Carolina State University website (available on the Internet Archive) referenced AP's rule as being to always use an apostrophe.

The 2009 AP Stylebook's "plurals" entry has no section on acronyms, but mentions "VIPs", I can't find anything addressing how to specifically pluralize acronyms. (The "abbreviations and acronyms" section is also of no help.)

Personally, I omit using apostrophes unless I can't avoid it. I do use them when talking about single letters or where it would avoid confusion. (For example, SOs for "Significant Others" looks like an incorrectly capitalized SOS.)

To paraphrase Carol Fisher Saller, the clearer usage is the correct one.

I agree with Wikipedia, wordreference and CMOS - acronyms and initialisms are "regular" nouns; plurals are formed by adding "s".

Checking Google Books for actual usage in a relatively "contentious" case, I searched for:

"OSs" unix windows linux 3120 written instances

"OSes" unix windows linux 1060 instances

"OS's" unix windows linux 520 instances

"Simpler" cases such as CDs vs CD's are even more decisive (over 10:1 in favour of the former).