Apostophes and Acronyms

I am never sure if I should put "s" or "'s" following an acronym. It has become my habit to add an apostrophe before my s, such as ABC's, CD's, CNA's, etc. I would guess that many of you may have the same question.
I would love to be able to answer your question but it seems that the jury is out across the board. WSU it was suggested that it is acceptable to use an apostrophe; however, Rutgers suggested that the apostrophe should be avoided.
Answer: find out your professor's preference. As for me, I am going to begin to leave off the apostrophe because it is really not necessary to have it there.