I use USSRs a lot,so maybe I can help.
I wouldn't say they stretch out,but they become more flexible,but in a good way.I can take mine now and stretch them out about 6 inches around and easily get them over my balls with plenty of clearance and then place them exactly where I want.If my hang is already under some tension with weight and other USSRs,this is very helpful,because fiddling with your hang,while it is under tension,seems to make me tighten up,rather than stretch out,which is what I seek,of course,when adding USSRs.This flexiblity also makes them much easier to remove during the course of an evening,if I need to.
I find for me anyway,I much prefer to place my USSRs above the weight,rather than below the weight.I find the rounded edges of the WMCs to be the most comfortable thing to be pressing on my balls,and I wear a fairly snug WMC,so I have NEVER had a pull through with one.The USSR,on the other hand,can flex and expand in diameter,so if I create a lot of mechanical force with them they can,and have,created a pull through.However,a pull through with a USSR is not a big,painful ordeal as I would imagine a steel weight pull through would be.
It seems that most guys figure on putting the USSRs on the bottom,directly against the balls,as a cushion,I just haven't found that to be comfortable for me.
Another thing I have found with the USSR,is if you are trying to loosen up a tight set of balls,after being out in the cold,is to stack a few on,then wear the top one as a cock ring,under the balls-over the cock,it seems to case a bit of a constriction that prevent the balls from pulling up,until they relax a bit.
Oh,and buy the 4-pack.Hope this helps.