Measure your pieces after you cut them.
Depending on your ruler and/or cutting method (using dies or a June Tailor strip ruler), the sizes may vary a bit.
When I cut around a template or a ruler, my pieces are probably about 1/64 inch (or more) larger than the 'designated size'.
Examples: If I cut all the way around a 4 inch template - the fabric size might be about 4-1/16 x 4-1/16 - so I can use a full 1/4 inch seam.
If I cut a 4-inch strip using a ruler in my usual way, the strip might be up to 1/32 wider than 4 inches.
I don't know what happens when one uses the June Tailor strip cutting guide. I've been meaning to check it out,but so far, I haven't.