Welcome to the board. You will learn so much here. It's how I start my day each morning. I always use the selvedge edge to square up my fabric rather than the fold. I know that the selvedge is straight of grain and if I lay my ruler line along that and cut off the excess sliver then I am working on the straight of grain. I guess I learned this from a lot of garment sewing in the past where when laying a pattern on I was taught to measure the grain line on the pattern by the distance away from the selvedge and when the top of the pattern grain line and the bottom of the grain line were the same distance from the selvedge the pattern was on the fabric grain like it was supposed to be. I haven't had any trouble with curvy strips doing it this way. I also press down on my ruler hard when cutting, moving my fingers along the ruler like a spider as I cut. This way my ruler doesn't shift.