I have had to remove wheels for various reasons (changing tubes, etc.) and dismounting the reaction arm, then having to re-mount it, make sure the rim runs straight in the frame, adjusting the shifting cable, and so on, is a PitA, and to be avoided if possible. You kids with your QR hubs don't know how easy you've got it.
Also, since tires have different profiles, I'd be dealing with the vagaries of those when swapping from another bike, rather than having an understanding of the underlying principles (which is to say, the actual diameters), which I have never understood. For most of my bikes, I have been able to re-used the rims that I got them with, and for some of the others (like the G&R), rim size selection was easy (650B) because there was no alternative that would fit. Live and/or learn.