[Fulltiming]

I would suspect that Newell is making that recommendation to maximize the current and voltage available to the slides to avoid overloading the motors, especially on the larger, heavier slides. It would seem that the same amount of voltage/current would be required to bring in a slide as to extend it.

Is it necessary, probably not. Is it a wise idea, I would think so.

As a good steward of your investment, Newell will recommend those procedures which they believe will reduce maintenance and increase longevity in the long run.