"SHTF" is an acronym for the "Shit Hits The Fan," meaning a natural disaster, a catastrophic breakdown in civil service, a military takeover, a New World Order, or an invasion by brain eating zombies that makes life an exercise in "every man for himself." (Also known as "The End Of The World As We Know It" or TEOTWAWKI--easily characterized as akin to a third NSYNC and Britney Spears tour.) Of course, depending on your view of the goodness (or lack thereof) of man, you may or may not consider a SHTF scenario likely. It is worth noting, however, that the New York blackout, the LA riots, earthquakes, and other fairly recent breakdowns in social fabric have all made the prepared feel pretty good about having a little SHTF ammo around. It all depends on your tin-foil hat quotient™.
Regardless of your politics, SHTF ammo is a good term to use to refer to ammo stored away (perhaps underground), "just in case." Criteria for good SHTF selections are obviously: Storage/durability, cost, defensive performance as an antipersonnel round, reliability, reliability, and reliability. This is ammo that--quite simply--just has to go bang every single time without fail.
As a general matter, new manufacture (i.e., less than 3 years old when you buy it) military-spec ammo is probably the best for SHTF use. The bullets and primers are sealed, they may have flash reducing powder formulas, they are loaded a bit hotter than commercial ammo, designed for storage under military (read: non-ideal) conditions, non-corrosive, cheap ($0.10 - $0.14 a round if you buy in quantity), and have good antipersonnel properties.
SHTF sort of supposes that you will be a lone actor, that engagements will be inside of 150 yards, and that you'll be in an urban or suburban environment. Of course, we tend to like M193 for these purposes. M193 has the added benefit of working in a wide variety of weapons and rifling twists, making it a good trade commodity, and flexible in whatever 5.56 weapon you're likely to get your hands on.
Opinion: Don't buy anything from late 1999.
Some purists might tell you that anything of late 1999 manufacture is likely to suffer from quality control issue because of the rush of many manufacturers to meet Y2K demand.
Opinion: You should probably avoid surplus ammo since there is no telling how it has been stored over the last many years.
Opinions (Pro and Con):
5.56 is not good SHTF ammo. 5.56 doesn't penetrate enough, it doesn't defeat light cover and the lack of fragmentation at range of FMJ rounds means that 7.62 is a better choice.
5.56 is best for SHTF. 5.56 is light, more of it can be carried, light recoil means faster shot recovery for follow-ups and it performs just fine inside of 200 meters. Since SHTF engagements aren't likely to exceed that there is no reason to use 7.62.