ORIGINAL: AWS
a little slack in one or both lines under tension is not going to have any impact in flight. the side that is tight is the one that is loaded, and as soon as it experiences any pressure the other side will start to get some slack anyway. my advice...leave it be.
I've noticed this too. And, I agree--leave it alone.
You'll NEVER be able to tension the cables enough to keep them tight in all situations. When you give right rudder, that side will get tight and the other side may get a little loose. So what? The right side is pulling the rudder over. Can't go anywhere, even if the left side is slack.
Just try and get equal tension on the wires when the rudder is centered and go fly it.