I know very little about how deraileurs work, so factor this in to my question. I want to have two rear wheels, the one I use now for climbing rides and it would be nice to have closer spacing on the cassette for primarily flat rides. If I change my rear wheel from the climbing to the flat ride (lower tooth count) cassette, is it necessary to do a readjustment procedure with 6870 (actually I have 6770 shifters and 6870 front and rear detailers)? Can you explain why if so or if not so?

I've used 11-25 11-28 and 11-30.... only 11-30 made me adjust the b screw.. for clearance.

Switching between wheel A and wheel B may require a slight adjustment, since not all rear hub spacings are exactly the same. If the 2 hubs are of the same model you should be ok, but different brands can vary a little bit.

When I switch wheels with my mechanincal group, I often have to turn the cable adjusting screw a revolution or so.

Ideally, there would be some very thin shims to put behind the cassette so you can make your wheels all the same.

Does anyone know of such shims ?

I guess I got lucky.. other then the mentioned B screw.. no other adjustments and now that I adjust the screw for the 11/30 is works fine with the other cassettes too.
Also 4 different brands of wheels.. Enve, Roval and Shimano and Bontrager.. SRAM(1190) and Shimano (DA and Ultegra ) cassettes

I always count on having to make slight adjustments when swapping parts, even like for like, due to manufacturing tolerances. When changing the number of cog teeth it is advisable to touch up the "B" adjustment to optimize shifting.