Apostrophes and uppercase

From TechWriter Wiki

Apostrophes are often (mis)used in uppercase words - most commonly where the word is an acronym - to indicate plurality. The basic rule is that an Apostrophe is used to indicate possession or one or more missing letters. However, this caused some problems with plurals in sentences that were shown all in uppercase. This problem was initially identified by newspapers (I believe, the New York Times), who were in the habit of using all capitals for news headlines, and who therefore took to using an apostrophe to denote plurals. So the headline "CDs: The Future of Music" would be rendered in uppercase as "CD'S: THE FUTURE OF MUSIC" because "CDS: THE FUTURE OF MUSIC" would have people asking where they could find this new CDS thing...

Some style guides now positively advocate this use as a mechanism for clarity, and allow it to be used on all words that are in uppercase (in their singular form). So "CD's: The Future of Music" is deemed to be the preferred approach, and the omisssion of the apostrophe is only seen as an 'acceptable variation'. This is back-to-front; the basic rules of grammar should be observed wherever possible, and flouted only where absolutely necessary for clarity.

This, then, gives us the basic rule of thumb that: An apostrophe should only be used for denoting plurality if the entire sentence is in uppercase.