I've done it both ways and ended up finding it made no difference. So I always store a bass fully setup and tuned to pitch in its case. It's just less work when I do finally crack it open and try to play it.
OTOH, the most time I've had a bass literally sit in a case completely undisturbed and unopened is only about a full year (one of my Rickenbacker 4003's). I haven't done the proverbial 20-years-out-in-the-shed thing, which would be a much longer term experiment that I just don't have the time to actually perform. My basses are either destroyed or stolen long before then, or I take them out and play them long before that much time elapses, etc.
I can't think of any bass made out of wood that would survive that length of storage without at least something going wrong with it somewhere on it. But I suppose it's possible.
As for drift with seasonal changes, I've also found that string tension makes no difference. Far and away, I have found this is affected more by truss rod type than any other factor I've been able to pin down. Namely, I find high-tech DA truss rods to really move around dramatically more when a cold front comes through, etc., than traditional TR equipped basses.
My Carvins (may they R.I.P.) all had high-tech DA truss rods with very fine tolerance construction, but their relief changed so much when the weather got cold that literally the next morning, the strings were all flat against the fingerboards. Almost 1/4 turn would be needed to make them playable again.
My L2000 CLF reissue has a DA rod in it and it moves all around when the temp changes. I just keep an allen wrench out all the time when the weather changes when playing that one.
My Fenders, OTOH, practically don't budge at all. A tiny tweak on my Tony Franklins from time to time, and the Jacos don't move at all.
My old '00 L2000 had the "bi-cut" neck with a traditional truss rod in it and I literally never had to adjust it.
Go figure....
Either way, I don't consider that a quality issue. My theory on why my DA truss rod basses varied so much was the very fine tolerances on a DA truss rod design - expansion and contraction of the two pieces of metal due to temp changes make large changes in the tension.
But that may be total BS, don't know. That's my WAG for why....
L