Lining up the torn edges - not the selvage edges - will give you a straight crosswise grainline. You should continue to stretch, corner to corner, to get the selvages closer to lining up, to also give a straight lengthwise grain.
The method most quilters use to get a straight grainline - folding selvage to selvage and sliding until the fold hangs straight - doesn't really work. This is what causes so many frayed edges and little pieces of loose threads on the back of blocks/tops.
Here is a video that shows how to stretch to correct the wonky selvage go to about 2:10.
https://www.youtube.com/watch?v=KF60qVY83j4