I have a feeling this one bugs me more than it bugs most people.
Letters written as letters should not be pluralized with an apostrophe and an s. It makes no sense at all to say you got all A’s and B’s on your report card, or that your daughter had better learn to mind her P’s and Q’s. An apostrophe has two general uses: First, it indicates POSSESSION of something, as in Please put the ribbons in Team D’s box. Second, it indicates the REMOVAL OF LETTERS in a contraction, as in I don’t know where I put it (in this case, the apostrophe indicates the removal of the O in not) or ...o’er the land of the free and the home of the brave (in this case, the apostrophe indicates the removal of a v).
People want to use apostrophes when pluralizing letters written as letters because they’re worried others might try to READ the letters and get confused. For consonants, this is an unnecessary worry, because readers are quick to see that Cs is not a word, and therefore should be read as “more than one C.” For vowels, I’ll grant that a sentence can be confusing if an apostrophe is not used, as in “Students who got As are exempt from this assignment.” One solution to this problem is to use italics, which one sees quite often. If you do this, italicize the letter but not the S that pluralizes it. Another way to deal with this is to rewrite the sentence, so you have something like Any student who received an A is exempt from this assignment.
I must confess that I have a near-daily conflict with this issue, because my favorite Major League Baseball team is the one that plays in Oakland. Long known as the Athletics (which presents a completely different grammatical issue), the team was officially called the A’s in the 1970s, ‘though the nickname had been in use for decades. Since the logo on the baseball caps features the A with the apostrophe, I commit a major act of hypocrisy every time I wear the team’s clothing. In my written communication, though, I almost always insist on referring to the team as the Athletics in order to avoid this awful situation.
Since it makes no sense AT ALL to use an apostrophe this way, I often wonder how the apostrophe got chosen for this misuse. I mean, why not a period, as in straight A.s? I know that looks odd to you, but it looks just as odd to me to put an apostrophe there. Thankfully, with the prevalence of wordprocessors comes the ease of italicizing, so if you stick to italics for those odd situations and nothing at all for the consonants, you should be FINE. Please stop with the apostrophes!