Andi has a great tutorial about this.
Here is what I do:
1) Make yourself several 2.5" strips, sew them together then press them open. They should measure 4.5".
2) If your sewing machine allows you to adjust your needle and if your sewn strip is less than 4.5", then move your needle a bit to the right, stitch and measure again -- play with this until your strips are the correct size and make a note of your needle position.
3) If your sewing machine does not allow the needle to move, use a piece of blue painters tape to mark your sewing line and adjust it until your sewn pieces are the correct measurement.
What happens is that your fabric, when it is pressed open, takes up some of the finished width of the fabric and the little adjustments compensate for this. When you reduce the width of the seam to accommodate the fabric, this is called taking a scant 1/4" seam.
Something else that can drive you nuts -- always measure on the same side of the line on the ruler. Decide what side you are going to use and stick with it because switching your cutting guide from "inside" to "outside" can make a couple of thread difference in the with of a strip and in a large quilt it can make you way off.