I have the Viking reload on talic seahorse stands. In order for the kayak to sit down on the straps properly and not have the metal of the stands pushing into the kayak, I have them positioned pretty far apart on the narrow ends of the kayak.
Will the kayak sag in the middle? If so, how often should I take it off the rack?
When I go camping or take it on vacation, do I need to worry about it sitting flat on its bottom? Will this cause deformities? If so, what is a reasonable amount of time to let it sit before repositioning?
I don't think you have an issue regarding the stands. Ideally, you'd position the stands dividing the kayak into even one-third lengths. I suppose an analogy is to think about a gymnast doing an Iron Cross on the rings: the farther apart the hands are (supports), the more pressure on the system toward the middle. So, the farther apart, the more potential for "sag."
While camping out, I'd just find a level grassy area and it should be okay. People (me, too) carry our kayaks flat in the backs of trucks with no issues. High temperatures in July and August will nudge us closer to issues I suppose but I think you are likely okay.