IINM, it has to do with the original diameter of lobe section of the blank used to make the cam, or how far they cut the minimum diameter of lobe, the part that doesn't make any contact.
I can testify from personal experience that junk yard shims would be a major waste of time, as said above, factory shims don't vary much. Odds are the IPD cam will need 8 new shims, all of the IPD turbo cams I've installed needed thicker shims.
The worst thing that could happen if you do not order a shim kit is that you need thinner shims than you have on hand, and thus you won't be able to measure the clearances, because the clearance would be 0 or below 0. There you would probably have to put the old cam back in for now and then find a shim kit, or buy a few smaller ones and try them out until you can find at least one that you can use to set each valve with, an exasperating process. The odds of this happening would be rare, all the after market cams I've installed used either the same/similar size shims as the factory cam, or needed thicker shim; where measuring the clearances using your current shims would not be an issue. But if going from and aftermarket cam to a factory cam, you might want to have a shim kit, or have the original shims from that engine somewhere on hand.
Just to make mention of a related item, I strongly recommend against installing new valve stem silencers until after you've completed all valve clearance adjustments, but then I would replace them after each adjustment.
I'm curious what are the problems one might find if a new cam was not broken in via the aforesaid method?