I was taught to get the straight of grain this way.......
Fold the fabric selvage to selvage. Then hold it out in front of you. If there is a distinct bump or fold along the folded edge, then manipulate the selvage edges by scooching them along, one in one direction and the other in the other direction, until the fold disappears and the fabric lays flat. Then even off the cut the edges with a ruler and a rotary cutter.
Works every time!