When the quilting distance is specified as no more than 5", this means 5" in all directions. Adding hand quilting so that no knots or quilting lines are further apart than 5" would be very helpful to the integrity of the quilt.
With polyester batting and cotton battings that are not needlepunched, I would knot at half the distance recommended for quilting lines (in this case, that would be every 2.5"). This is because individual knots do not provide anywhere near the same level of support for the batting that quilting lines do. Knots need to be closer so that the batting will not pull apart with repeated washings.