I agree with Tartan - it depends. I am a prewasher so to cut for piecing across the width of fabric, I first press everything that I've washed. I then match selvage to selvage but then sort of wiggle those edges to get my fabric as flat as possible at the fold. This helps ensure getting the piece 'on grain' and avoid the dreaded 'v' when cutting that folded edge. I then square up one edge by placing my ruler on the folded edge and cutting a new straight edge. I then use that cut edge to measure and cut all my pieces.
If I'm doing length of fabric cuts for borders, I'll usually tear those pieces. I'll rip them about 1/2" wider than required so I have room to flatten out those wonky edges and trim up a bit.