Fabrics that arrive on your worktable with the weft grain at anything other than 90º to the selvedge have not been diligently finished or dried. The truism is that the more you pay the better aligned both grain and print will be.
When a fabric does arrive out of line it is possible to re-align it by washing and stretching it back to shape. To do so without a tenter, pin, or bulldog clip, the fabric securely to the worktable in a warm atmosphere or somewhere with a gentle breeze until it has thoroughly dried. Note that the fabric will not willingly return to square–you’ll need to be in charge