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MarkThe curvature of the card itself is not an issue. But you'll need to keep an eye out for damaged corners and occasionally edges. You also need to be careful not to cause too much surface damage when sorting, as cards tend to stick together. Lift rather than swipe each card from the card below to avoid surface scratches.
During the late '90s I broke open five 1985 Topps vending cases. This was during the period when McGwire PSA 10s were selling for up to $7k on Ebay. You'll need to be exceptionally careful when pulling the cards from the boxes and sorting through them. I removed all the cards from the vending box and then sorted through them in small stacks of about 50 cards at a time.
My experience submitting 100 McGwire cards to PSA was about 60 came back PSA 7s & 8s. I had about 30 PSA 9s and about 10 with serious corner/edge/surface damage in PSA 6 or below range. Unfortunately no PSA 10s... and come to think of it I've never had a PSA 10 come out of any vending box.
Good luck!