The tension you want is one that 1) has preferably no more than ~1/2" of play in the top with the bottom tight and (far more important) 2) never goes tight as you spin the crank for enough revolutions to get the every combination of crank orientation and wheel orientation. This is because neither cogs not chainrings are concentric. Both have machine tolerances. Better quality stuff is better, parts made fro the track better still in general and the best track parts are very good, but still not perfect. The most important aspect of chain tension is that too much, ie a tight chain puts a huge load on your bottom bracket and hub bearings and will shorten their life dramatically. Too loose simply means that you may drop the chain on bumpy roads. Now, if you choose to ride without brakes in hills on cheap drivetrains, it could be argued that killing bearings is cheaper than throwing chains on steep hills. (I'm too old to be making those choices.)

Ben