I spent 7 years working for a defense contractor. Four of those years
were in the manufacture of torsion bars and other suspension
components. The engineering behind torsion bars is that they will
only operate in a range such that permanent characteristic changes
(plastic deformation) will not occur. Unlike coil springs which can
relax if left compressed, torsion springs will never loose their spring
unless they are exposed to overstress conditions. As I have seen
TM's from the early 90's that have not had a problem, I would
surmise that they are properly engineered and whether left up or
down, no change will occur. Torsion bars are used in many
applications where the weight of the vehicle is always loading the
spring (Chevrolet trucks, Volkswagon cars, Bradley fighting vehicle).
They fail when they are cycled through a range of motion many
hundreds of thousands of times. When the do fail, it is a fracture
across the entire cross section of the spring (i.e. it breaks). IMHO,
the torsion springs are nothing to worry about. Whether left open or
closed is a matter of personal convenience.