The importance of cutting accurately, and being uniform in the method you choose to cut.
There are different ways to cut, generally speaking the most accurate cutting does not utilize the grid lines on the mat, rather the grid lines on the ruler only is used. I use a two ruler method as follows:
1) Use the smallest ruler possible to cover all of the fabric you need to cut. This is your "cutting" ruler.
2) Use another ruler that covers 2/3 of the other ruler (or fabric). This is your "measuring" ruler.
3) Lay fabric flat on your cutting table with the edge to be cut to the right hand side.
4) Lay your cutting ruler along the right edge of the fabric with a little bit of fabric showing to the right of the ruler all the way down.
5) Cut along the ruler, this is your "straight line" (note - if you are working with directional fabric, you may have to finagle the ruler placement to get a straight edge that matches the print on the fabric).
6) Slide your cutting ruler to the left of the straight edge.
7) Using your measuring ruler, find the measurement you wish to cut. Place the ruler on the fabric to the right of the cutting ruler, and slide the measuring ruler to the right until the measurement you want is on the straight edge. You will want the right edge of the measurement line to just cover the straight edge.
8) Slide your cutting ruler to the right until it butts up against the measuring ruler.
9) Keeping you left hand on your cutting ruler to keep it in place, remove the measuring ruler with your right hand and pick up your rotary cutter.
10) Again, keeping the cutting ruler firmly in one spot, make your cut on the right side of the cutting ruler.
By staying uniform I mean ... always measure with the same ruler (or brand of rulers), the cutting ruler doesn't make a difference. Also, make sure you always measure the same way .. ie make sure the measurement line on the measuring ruler is always in the same place on the fabric when you measure.
I learned the above cutting instructions from Sally Collins, a quilter/teacher well known for her precision. I highly recommend her book and video "Mastering Precision Piecing"