[boxstershorts]

Two things:

Material outgassing, aging, drying out: This happens are a different rate depending on humidity, temperature etc. Materials shrink when this happens although hardly noticeable over the course of a few weeks. Better to shrink with top up than down.

Flow: Materials tend to relax to relieve stress over time. If in compression, in tension, in bending, they will permanently deform and lose their elasticity in local areas. This will permanently deform your top. Better for it to happen when the top is up than down.

I recommend to store your car with top up (fully stretched) for the top to last longer.