I hate cutting with rotary blade when it is a longer piece. I have a ruler I bought at Wal Mart for $30. You can not always find it there and the quilt shops say they won't sell it because there is no profit BUT! if you have trouble with wobbly/wonka cutting this ruler is great. The blade is ATTACHED to the ruler! You can't cut "off". It's also a 24 inch ruler so yeah! Large pieces! It's made by Fisker's. I would not trade it for the world!
See if you can find one because I got it when I was a new quilter because the rotary really scared me and with this WOW! I have been quilting about 10 years and still go to it first.
Also check your quarter inch on your foot for sewing. If you are squaring your blocks it's probably that!