If you want the lines in the plaid to appear parallel to a straight cut edge of your patch, cut the plaid fabric off grain, on print. The way to do this is to align one of the ruler lines with one of the lines on the plaid. You may waste a little bit, but it's worth it if you're after a cleaner, neater appearance.

Yeah, the only plaids I will work with are woven plaids, like from recycled shirts. I know it's a 'look' to cut them every which way, but I have to cut on the plaid. Scissors do work best for this.