Some stores will rip fabric to get the straight line this distorts the edge and is not a good idea. You can line up fabric to cut off by; hold fabric up with one selvedge at. The top. Lift up the second selvedge take no notice of the centre fold. Keep sliding the selvedges along the top( keeping together) until the fold is square. Now lay down and gently cut level.
Some fabric has a terrible waste. Cutting across your way may alter the joining of some blocks. One direction has more stretch than the other, not sure which is which.sorry.