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I haven't experimented with reduced loads in many years, but I do remember in an old Lyman manual of the day that if it is a bottlenecked case which headspaces on the shoulder, then you will slowly begin to see the shoulder recede, which can then lead to case head separation when you use reduced pressure loads...
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Good point, I believe the case headspace shortening is the product of the firing pin blow driving them forward, in effect sizing them in the chamber, without the normal pressure load ironing them back out to full size. However, I believe the main problem would be using the headspace compromised cases for full power load after they had been used for enough light loads to be a problem. I may be mistaken, but it seems that light loads dont have enough power to stretch and separate a case. Alternating cases between full power may help to keep them ironed out, I dont know, or simply keeping cases used for light loads separate. I haven't seen any warnings about it in newer Lyman manuals, even the cast bullet manual, which has many loads at far below normal pressure level.
A case headspace gauge may be useful in keeping track of the case headspace question.
Belted cases should be the same as rimmed regarding the shortening effect, if indeed its still considered to be a problem.