Most systems will have a little slack on one side during full deflection. Unless your geometry is perfect this will happen. Most times it's not a big deal. When the plane is flying the wind will keep tension on the deflected side. Have you tried crossing the wires? This will help under some circumstances. As far as tension when centered. To much is worse then not enough. To much will wear out the servo and could cause a drain on the battery. Wearing out the servo is the biggest issue.

David