There are different schools of thought on this. Here is what I do:
Working from the top: rubber washer right up against the bolt head ( NO METAL WASHER ); this now goes through the bottom of the tank. Now I put a metal washer and then a nut. No rubber washer on the underside of the tank. I can snug this nut up real tight to ensure a good seal inside tank, and this can be tightened without fear of cracking porcelain.
Now, the tank gets set on the bowl. I put up a rubber washer, then a metal washer, then the nut ( which may be a wing nut or regular hex nut.) This nut gets hand-tightened ONLY. NEVER a wrench here. I push down gently on one side of the tank and then the other to allow finger tightening and proper leveling of the tank.