always. a scrap 3/4 x 1 or so.
I'm not yet organized enough yet to have the precuts for a "future" quilt to use as leaders.
When I'm chain piecing, I leave a tad more than 1/4" between the units. I clip in between, leaving both with a bit of tail.
When units opened & sewing next seam, the little tails tell me where the seam is. Right now, I'm sewing strips of 8 of 1.5" stamps. so, opened up, I can see if the two squares I'm sewing together are the same finished width, and adjust seam allowance accordingly.