One of the first classes I took for quilting was how to rotary cut.
1. When rotary cutting strips, never cut more strips then the width of the strip without re squaring up the fabric. For example: Cutting eight 2 1/2" strips, after every two cuts, re square up your fabric.
2. When lining the ruler measurement line to the fabric keeping the full width of the measurement line on the fabric for every cut will eliminate the need for using a scant seam allowance.
3. Fabric needs to be pressed and if folded, the open fold should be perfectly aligned to each other like a seam.
We practiced cutting by drawing lines on the fabric and cutting leaving the line on the fabric. This way we could see where our cutting was veering off or where our ruler was moving. I still draw line on fabric when I'm having an off cutting day. I use the Gypsy Gripper on most of my rulers. It's a must for the long ones. One slip of the ruler can waste a lot of fabric.