I use standard comments for different situations. An older home with two-wire ungrounded branch circuits isn't considered to have defective wiring, and neither is the absence of GFCI's in this case...well, maybe in Texas! And, exterior locations in, what, '71? Then baths, then kitchens, then garages, then laundries, etc, etc. It's safe enough and surely simple enough to strongly recommend it as a safety upgrade in all wet locations, and the receptacles are relatively cheap. They work on the two-wire systems, too, as we all know (I think). I go one better: I recommend them upstream in bedroom circuits, to give a measure of added protection without having to make expensive wiring upgrades. Kind of a poor man's AFCI, if you will. There is certainly nothing that says you can't, and if you happen to be buying a home with a two-wire system, it's not a bad idea at all. And, most beautifully of all, it doesn't tick off the Realtor Gods.