I personally have never been able to sew a perfect block, with perfect seam allowances, which effects how the whole finished quilt top sews together, as each 1/16th of an inch can add up to a significant amount in the finished size.
What I came up with early in my quilting, was to draw a line at the 'finished size', with a mechanical pencil, on the back of each unit or block. I use the line as a guide to pin to the next block, plus to sew along. The final result may have the blocks slightly off, or 'charming' in the quilt top, but the final quilt top itself lays flatter and in the end the finished quilt is nicer.
The 'Blocking' how to page, that describes the technique step by step, is available by request (free) here.
I start with a fine grade piece of sandpaper and tape it to a cardboard with masking tape.
Draw a 'sewing' line, with the mechanical pencil, around the ruler.
This gives you good corners and makes up for any uneven sewing. You can trim any extra seam allowance, if you wish to take the time.
Pin your blocks together using this pinning trick:
Place a pin in each corner of both the two blocks you are joining. Don't anchor them. Hold the pin perpendicular (90*) with your first two fingers (underneath) and your thumb on top. Now place an anchor pin (going in at an angle) beside it. Do the same for the other corner, and a few times in between, making sure you are matching up the two sewing lines. This pinning method keeps the blocks from shifting out of place.
When you combine both of these tips I promise your quilt tops will end up neater.