I suppose any hard and fast rule is not warranted. Consider that others argued that acronym and abbreviation are not the same thing:
"There is a difference between acronyms and abbreviations. An acronym is usually formed by taking the first initials of a phrase or compounded-word and using those initials to form a word that stands for something. Thus NATO, which we pronounce NATOH, is an acronym for North Atlantic Treaty Organization, and LASER (which we pronounce "lazer"), is an acronym for Light Amplification by Stimulated Emission of Radiation. FBI, then, is not really an acronym for the Federal Bureau of Investigation; it is an abbreviation. AIDS is an acronym; HIV is an abbreviation. URL is an abbreviation for Uniform Resource Locator (World Wide Web address), but many people pronounce it as "Earl," making it a true acronym, and others insist on pronouncing it as three separate letters, "U * R * L," thus making it an abbreviation. The jury is still out. (I vote for Uncle Earl.)
It appears that there are no hard and fast rules for using periods in either acronyms or abbreviations. More and more, newspapers and journals seem to drop the periods: NAACP, NCAA, etc. Consistency, obviously, is important."
Consider also that the article "the" would be pronounced differently as in "da" and "de" when it precedes different words. We do not spell "the" differently however. It would seem logical to use 'a' in writing except in the most obvious sitaution and leave how it should be pronounced to the reader depending on how s/he chooses to pronounce the word (be it acronym or abbreviation) that follows.