The guy at your shop is absolutely right, 50# is plenty to kill a deer, but here is why I shoot 70#. I want to shoot the heaviest draw weight that I am comfortable with because I want my arrow to get to that animal as fast as possible and with as much bone crushing energy as possible. I want all the speed I can get to provide a lesser chance that that deer will jump my string, especially at the 30 - 40 yd range.