Okay, as a relatively newbie (only my 5th year of quilting), I had the same issue. And yes, the thread can make a significant difference as you make more blocks. So, I sew sample piece putting my fabric on the 1/4" line on my my machine using the fabric and the thread that I intend to use. I sew it and iron it (both kinds of seams to one side and open depending on the pattern I am going to use) and then I measure to see how much I am going to have to adjust the seam size. I have found when the thread is thicker and sometimes the thickness of the fabric, an adjustment needs to be made. So, I measure from the inside edge of the thread to the outside of my seam and if the seam is larger, I try again until, I get an exact 1/4" from the inside edge of the thread and the outside of the seam. Then I put a piece of painters tape, the seam allowance attachment with the screw tight (older machines), or any of the new seam allowance attachments that will keep my seam exactly at 1/4" and making sure that little seam doesn't get away from me at the very end when it tries to make a smaller seam. Steady as it goes. I am getting better at this quilting thingy as I learn from all these great people on the Quilting Board!!!!!!!