Canadian, while I don't disagree that you haven't had any problems with your system, I don't believe it's because you kept your summer tires near the truck. The sensors themselves go to sleep in order to save battery until they detect motion.
Since you only have one set of sensors programmed to your truck, the computer remembers the code(s) when you put them back on in the spring, no problem. Even if they didn't take right away, you should be able to re-initialize the sensors with a magnet come spring.
Actually, I think you'd have more trouble if you had a second set, because then you'd have to "switch" the computer when you changed the rims, unless the computer can recognize more than four wheels at a time. All you're preventing is having the light on your dash for a couple of months.
You'd have to talk to a tire shop, but I don't think they can force you to install sensors in your winter wheels. I know the system cannot be disable because it comes that way from the factory, however there should be nothing that says you have to continue to use it. If you wanted to safety it, then maybe, but as long as you're driving it you should have the choice, IMO.