I agree it really shouldn't matter as long as they are stored in a dry semi-clean area (free of dampness, bugs, mice, etc.).
I figure if you store it pressurized you are placing unnecessary pressure on all the o-rings. Removing the lid, posts and dip tubes allows the rings to maintain a round configuration versus being pressed flat during storage.
I've just started the kegging process, but I won't place the o-rings on until I sanitize and fill the keg. It's just more logical that way.
i do the same. i find that liquid/water gets trapped in the liquid tube sometimes, so i have to put the keg right side up for a bit to drain it, then invert again.