Punctuation of abbreviations
This is an area where ‘the rules’, or at least the expected conventions, are changing. In the early part of the twentieth century, a full stop was used after abbreviated words – unless the last letter of an abbreviation was the last letter of the word, as in Mr as short for Mister, and Dr for Doctor. The practice since the advent of the computer has been much looser. Time and motion studies have encouraged organisations to omit the unnecessary full stops that used to fill minutes in the day of a typist. Acronyms like NATO (for North Atlantic Treaty Organisation) and short titles like BBC are rarely seen now in their traditional forms of N.A.T.O. and B.B.C. (Oxford Reference Online Premium gives (March 2010) the information that there are six conventions for writing abbreviations, committing itself to no decision on which to use ([]).)
So unless you have an editor or a teacher who maintains the old-fashioned convention, don’t bother with full stops in abbreviations of terms in organisations, techniques or machines etc that are used in your subject area. In medical circles, the quick way of referring to an electro-encephalograph nowadays is ECG - pronounced as three separate letters ('EEE SEE JEE'), but written with no full stops. It may sometimes be better to use the stops where the words without them can be read ambiguously: in common school slang, a 'swot' is someone who works too hard (or is thought by his fellows to work too hard), so the abbreviation for the name of a type of analysis in Management S.W.O.T. (= Strengths, Weaknesses, Opportunities and Threats) is better written with the full stops.
Note that conventions in the area of Bibliography may be different. OED, like most publishing houses, uses full stops after the abbreviations for titles of books. After abbreviations like v (for 'volume') and p (for 'page'), practice varies. Check the House Rules of your publisher or Department for guidance.
In the older style, envelopes always had many full stops – and commas. Schoolchildren in the 1950s used to be castigated for the omission of the stop in Ave. – and for its inclusion in Rd, which led to heated debates about whether the ‘t’ in St (for ‘street’) represented the first ‘t’ or the last. My own teacher was quite clear, not to say firm, that in Ave. the ‘e’ was the first in that word, not the final one, and that therefore the abbreviation should be written with a full stop. This is how pedants are made!
See also postal address, another area where rules of punctuation have changed.