I doubt you can get the tank and lines completely dry as a method of preventing contamination. If you could get ALL the water out then you would not have to winterize. The drain valve and tank are not designed to completely drain the tank, let alone the plumbing. You might get close using compressed air however.
According this this web site
, RV antifreeze in the proper mix does have some ability to prevent tank and plumbing contamination. But if the concentration is too low it actually encourages the growth of microbes.
In addition, RV antifreeze lubricates the plumbing and helps to prolong the life of seals.
So, concerns over wet feet aside, it probably makes little difference if the drain is open or closed during winter storage. I would close it just to keep insects out.