I am not convinced that some loosening does not happen, but if it does happen (and seemed to happen to my 243 ultra) it happened fast, first hundred or so. But I don't think it is the frame that is stretching but the barrel lug wearing in or compressing.
My guess is that if the notch in the lug is not perfect flat with the hinge pin the bolt thrust will ever so slightly deform the luguntill there is enough bearing surface to resist the force. Granted we are talking about small amounts but my 243 came home tight, but soon developed a "rattle". It took me a while to realize that there was excessive play between the notch in the lug and the hinge pin, so much so that the gap at the bolt face was over .006". To get rid of that rattle I had to shim the lug by .004", and it shoots OK now and shooting good or great is still a possibility.
I have yet to shot my 204 but that will change by next weekend, and this time I will be keeping a close eye on it as it wears in. I sill have plasti-gauge left so I will have before and after reference points. But I don't actually expect that it will be problematic.
First off the Marlin takeover seems to have had a positive effect on overall quality(just comparing my 2 handi's) and the 204 should have much less bolt thrust just for the fact that the case head is so much smaller.