I always ask for extra boards although I never took the open source option. I have stopped using their PCB service over a year ago and switched back to seeedstudio. Seeed often gives one sometimes 2 extra boards for my 10 pcs or 50 pcs services. For 100 pcs service I may have got 5 extra once. If you're just having fun with designing PCB, the extra is nothing. If you are selling kits with your designs, they bring some extra cash so I ask every time (they probably don't consider my request but would add a board or two if they can use the extra to fill a panel) 