Stanley had a patented method to temper irons (I believe it was in a molten lead bath), which allowed them to be very reproducible in tempering. For this and other reasons, you generally can not go wrong with older planes. You will always have to do some work on these planes (cleaning up, removing rust, etc.,) but in the long run this is a great learning exercise to see exactly how your plane works and to learn how to fettle it. In an emergency, I did purchase a big box store Stanley plane and on the first stroke, the cutting edge of the plane rolled up, much to my surprise.
Mr Anderson has it spot on! Molten lead baths were indeed used to temper Stanleys cutting edge tools. Not only the plane irons but chisels too. I have met a couple of old timers here in Sheffield who remember carrying out this procedure. They also mentioned that the steel quality of the older tools was much better than is mass produced now.