[Rob Colatutto]

Better tensioned chain will have less backlash when you are switching from driving forward to driving backwards. Wether its tensioned or untensioned you will get similar moments on the chain and run the same risk of snaping the chain in either case. Preventing the chain from hanging down is not something I'd suggest doing and as Tom said, it makes it easier for the chain to come off your sprockets. The precission you need in lining up sprockets also varries for different chain sizes as does the chains strength.