This. ^^^^^^^^
The rolling process creates directional strength differences. The sheet is stronger along the length than across the width, basically due to stretching during rolling in that direction aligning the microstructure along it. Think of a fibrous structure as an analogy.
The effect can be reversed for a more uniform length versus width strength by annealing. Cold rolling increases it.
With the aluminium sheet you are likely to buy the differences will be negligible and probably not noticed. Repeated bend/unbend should be avoided as that does weaken the metal in the bend area.
Have you been to the MMC factory? The panels are cut to a template and fitting is done with hammers (panel beating types), shrink forming also in situ on the car with filing/sanding to get the desired surface finish. What you will be doing bending sheet will likely be less of an effect than panel beating.
