You don't HAVE to plug the fingers. I have had manyballs where the fingers met, but at the bottom, 1.5" down, and the bridge never cracked. At that time, it was intentional as I had the driller put the holes at 0" pitch. Not one of those many balls cracked at the bridge.
Of course, mine met 1.5" down, not .5" down.
Why not experiment and leave them as is. See what happens. You can always plug the fingers if they do crack.