Leaving a double bass lying on its side for extended periods can result in what is referred to as "lambda creep." This is something that affects carved tops only. Because the gravitational forces are perpendicular to the grain when a bass is lying on its side, microscopic compressions of the grain can occur leading to a more "closed" sound when the bass is returned to its normal position and played. The bass can be "re-opened" by bowing extra long notes under room-temperature conditions with normal humidity. Storing the bass upright avoids lambda creep but, again, gravitational forces come into play leading some to suggest that half of the time the bass should be stored upside down in a stand. So far, no manufacturer of which I am aware is supplying a stand that can accommodate that orientation. In the case of laminates having plys of alternating grain orientation, it is best to store the bass upright and on its side for roughly equal periods as this distributes the lambda creep fairly equally among the plys. No doubt, many here are reading this in bewilderment. Rest assured that this is complete nonsense. Given some of the claims made here on TB, I'm betting you just had to wonder.