I just finished a string X (from quiltville.com) and started on the foundation paper....and ended up about 10 blocks in realizing that I didn't need it. But I was sewing straight seams and in the end just cutting the 4 corners on the bias. I just sewed strips together until I got at least the length I needed (10 1/2 inches)...pressed and then trimmed...But I also used at least 5 inches when in the end they all trimmed to 4 1/2. So I had some wiggle room to get them straight after sewing.
I think it would matter more if you (1) had smaller strings than I was using or (2) were going to sew them diagonally...it would make it easier, I think.
eta: Sometimes it is truly a personal preference and sometimes it is because it truly needs it....on mine, I found it to be a preference. But I tried it first to see if it made a difference!