RE: Level
I recommend to all my customers that they use a level at shots over 25-30 yards. On an average, there will be an 8" left or right difference at 40 yards if you're off level by 1/2 bubble. The difference depends on how far apart your sight is over your arrow. The higher your sight, the more differnce there will be.
You would be surprised as to how easy it is to 'cant' your bow on uneven ground or treestand. It can become more of a problem if your bow isn't balanced/stabilized properly. We spend a lot of time getting our customers' bows balanced/stabilized.