If an aluminum truck cap sits unclamped on something (for instance 2x4's), do the seams start to separate, does the cap get stretched out? I have seen this on fiberglass, wondering about aluminum. Thanks.

 

Not in my experience.
Be aware that seams and screws are sealed at the factory, but they don't last indefinitely. You need to inspect and re-caulk seams and screws at least every two years to maintain water tight integrity.