I've run across some "ATX" supplies that DO require a load, but as you say, they may be "older" types, and not be built according to the latest standards. If the OP uses an LM338 type regulator, most of the manufacturers DO say that additional capacitance (1µF t0 1000 µF) on the OUTPUT will improve transient response.(Surprise?) I would suspect the same for most any power supply. I can USUALLY tell just by weight alone if a "wall-wart" is a switcher or a transformer/rectifier (linear?) supply, and I haven't run across any transformer-type wall-wart or laptop supply that is rated for much more than 1 Ampere, making such "wall-wart" or laptop supplies unsuitable for the OP's use or needs.