Why narrower than ¼ inch
Pressing a seam to one side folds a tiny sliver of fabric over the stitching. One seam loses almost nothing, but a block with many seams can finish noticeably smaller than the pattern says. Sewing each seam a thread or two inside the ¼ inch line gives that sliver back.
Test your seam
Sew a test, press it as the pattern directs, and measure the joined fabric. Adjust the seam guide or needle position in small steps if the pressed unit is too narrow, then test again. A good check is to sew two 2½ inch strips together: pressed, they should measure 4½ inches across. See seam allowance and finished and unfinished size.