It’s more to do with the alignment of the chain than the tension. This is why
the current recommendation is to run the chain across the top beam, you don’t
have gravity pulling it back out of alignment.
if you have too much tension on the bungee cords, you will end up having more
tension on the slack side of the chain than on the sled side. When you
transition from the majority of the tension being on the sled side to the
majority of the tension being on the slack side, there will be a bit of motor
movement that does not move the sled (as the backlash is taken up), resulting in
an accuracy loss)
If you keep the chain aligned with the angle of the sprockets, you just don’t
have skip to worry about. When they are badly out of alignment (and the roller
chain specs allow for very little misalignment), at the very best you get
extra wear on the chain and sprocket, and you have a really bad fight to prevent
sticking.
David Lang