If you ordered UGCs, I would NOT recommend printing a bunch in advance. It's well documented that each and every batch of UGCs are a little different in size. That's not to say the cases themselves are different sizes, it's just that the seam on the sleeve is not exactly the same every time. What might happen is that you could print off a bunch of covers in advance and then they don't fit right when your cases arrive.
I would recommend waiting until your cases get there. Then print off one or two covers and test fit them to make sure everything works. You might need to make them slightly smaller or just trim off an extra mm around the edge. The good thing is that all of the cases in one order will PROBABLY be the same. So once you get one cover to fit right, the others can probably be done the same way.
Hope that helps.