They would be, if the fabric were straight. Often it's out of kilter and needs to be gently stretched back. We learned to do this when I was a kid learning to sew in home ec class. This article tells what is happening and how to fix it - not aimed at quilters, but it's still fabric.
http://www.american-doll-outfitters.com/grain_line.html
hah- forgot to post the link the first time. Thank goodness for the ability to edit posts.