In general I agree with you, Bearisgray, but it depends on the issue. I'm still a big believer in prewashing for all sorts of reasons but I also recognize practical considerations especially in precuts. In shrinkage for example, I think the problem is magnified by the number of repetitions. So like if our seams are off just one thread it isn't that big a deal, until you double that by one thread each side and 100 repetitions over the length of a quilt it can add up. So yardage and width add up but that 2" square that is stabilized by our seams and the surrounding fabrics might shrink a teeny tiny bit but not enough to cause trouble -- as a 2" square. As a 12" alternate block, there may be more issues.
Likewise, for dye running, most of the time that isn't a huge issue. Yes, I've had the rare really bad experience but that has just happened only once in my career -- of course, I went to great lengths to make sure it didn't happen again!
But I like working with batiks with black as a neutral instead of white partly because of the potential for dye running even though I have tested and prewashed my fabrics.
I have a large stash and so it's sort of easy for me to not buy precuts. It's not so easy for other people to afford the variety they want at the prices we have. Again, I just can't see me ever prewashing jellyroll strips, I'd cross my fingers and hope for the best and see what I get after the top is made.
So I'm not so worried if the combos are of similar colors/values, mostly if there is high contrast between the lights and darks. But something to keep in mind is that if the lighter fabrics are also printed (as opposed to solid white for example), chances are they have been treated with dye resists or other treatments in their manufacturing process. That's why they tell us they worth the higher prices anyway...