IM(NS)HO, for most quilters the problem is the pressing, not the cutting.
As an example, let's say you are sewing two 2.5" cut pieces into a 4.5" unfinished piece. If you *carefully* put a name brand ruler so that the middle of the 2.5" mark is *exactly* on the edge of the fabric, you will get something the right size. (Assuming you haven't sliced away the edge of your ruler or anything). If you aren't following the edge of the ruler for some reason, you'd get something bigger, not smaller. There's a common saying that every ruler measures different, but I've put a lot of rotary cutting rulers by different makers up against each other and never seen a noticeable difference.
Next is sewing. If you are sewing *exactly* on the 1/4" line, your block will end up a little smaller than they should. Why? Because the thread itself has some thickness, as does the fabric when you fold it back to press. If you are using a 1/4" foot, cut some scraps and learn where the fabric needs to be in relation to the foot to get the right placement.
Finally is pressing, something 95% of quilters out there have trouble with. You can cut perfectly and sew perfectly, but if you haven't pressed it correctly, the block will end up too small. (Or two big if you stretch the
*&#% out of it!). I could (and have in the past) write a pages-long essay on pressing, so I'll distill it down to a couple of points. One is take your time pressing. It's not a race, and it's critical if you want precision. Two is, like sewing, it's worthwhile to train yourself how to press to the correct size. Mark the unfinished size of your block on some muslin and pin it to your ironing board. Then use that guide to teach yourself how you need to press the block to make it the right size -- not too small, and not stretched. If you want precision, you'll be glad you did.