ripping the fabric just follows the grain. You still have to get the fold straight. I always had a problem with curves UNTIL I ran across an article about how to do it right and I've never had a problem since. If you hold a piece of material up with both selvages even and together in your hand and look down at the fold..it usually is kind of wonky. But if you slide the selvage until the fold is straight and smooth but your selvages are off kilter your cuts will be straight. It doesn't matter if your cutting a big piece of fabric or just your strip. Get that fold straight! And allow a little extra on each side to get a straight cut from the shifting. I read about it in this book by Fon's & Porter....
http://www.amazon.com/Basic-Guide-Ro...9492209&sr=8-3
it changed my love affair with my rotary cutter. LoL!