
After checking several dictionaries, it generally seems (with the exception of the CADE) as if the standard, approved way to write it is firstly, as "dos and don'ts" (MS DOS?!), with "do's and don'ts" more a variation that is listed only secondly (or not at all in e.g. COBUILD3).
I presume that most of us would however prefer to write "do's" (it makes more sense visually), even if we might start seeming like those greengrocers (not greengrocer's!) who scrawl "Banana's - 50p a pound" on their scraps of cardboard in the marketplace.
Interesting eh (said in self-deprecating ironic voice)...ah well, look on the bright side, although you can't ever get back those two minutes you just lost in reading this, at least you now have the satisfaction of knowing why this thread is titled "Banana's"!