The Manual of Style guideline "Abbreviations" governs acronyms and initialisms and when to use or avoid them on Wikipedia.
Its basic advice is to write to be understood; opting for the full word or phrase on first mention, especially for proper nouns; favoring correct usage over Wikipedia's own guidelines; maintaining consistency within a given article; and not changing usage within a quotation.
The guideline includes a list of proper names and common initialisms which do not need to be spelled out on first use, such as: AIDS, AM, BBC, BC, CD, DVD, many in between, and USB. It also includes a list of useful abbreviations in common use across Wikipedia: These include Ave., Assn., Blvd., Mtn., and PhD.
What about page titles? It's not always clear. NASA is "NASA" but attempts to move "Central Intelligence Agency" to CIA have proved unsuccessful. Remember: the only absolute on Wikipedia is consensus.