I suspect that many folks not getting blocks the correct size may not be cutting accurately - that was one of my issues. It's really important to to have the measurement line of your rule *on* the fabric, not right beside it. (I like to have it about one thread in from the edge.) Once you are cutting fabric consistently, the thing to do is sew some test pieces and figure out where exactly the edge of fabric need to be to get the desired *finished* size, and then put marks or a device on your sewing machine so that you can consistently sew with the fabric there. (I like to use about 2 mm of post it notes taped down.))
Remember, there's nothing really magic about the 1/4" seam allowance - the thing to focus on is the finished size. (So, people who consistently put the measurement line beside their fabric (instead of on it) when cutting, and then use an appropriately "scant" 1/4" seam allowance may be able to achieve the desired finished size.) The tutorial that PaperPrincess posted a link to is a good way to determine whether you are sewing too large or too small of a seam allowance.