Consider ‘Mr Smith’ as opposed to ‘Prof. Crumb’. My rule of thumb for punctuation around abbreviations is that if the abbreviation ends with the last letter of the original word (in the case of ‘Mister’ to ‘Mr’) then no period is necessary. Conversely, since ‘Prof.’ ends half-way through its originating word ‘Professor’, stick in the dot.
(I’m not saying this is the absolutely correct way to do things; my philosophy for writing style is that consistency trumps, always, ad hoc style ‘rules’.)
I recently realised that I wasn’t following my sometimes-period-after-abbreviation rule consistently with ‘vs.’ for ‘versus’. I went to write ‘LaTeX vs. MathML’ and had to stop and think for a minute. Should I be writing ‘LaTeX v. MathML’ instead? Or just drop the period?
My other rule of thumb for formal writing is to avoid abbreviations entirely, and hence write ‘for example’ instead of ‘e.g.’ and ‘versus’ instead of ‘v(s)(.)’. Maybe I should adhere to this rule now to avoid having to decide what to do with ‘vs.’.