I've always used straight unless I had curves also. However, I am seriously thinking about doing more quilts with curved corners. I hate mitering. I think if I am using scrappy binding I will have to do straight of grain but if I'm using yardage, I will try bias instead.
Here's a good tutorial for cutting bias strips fast:
http://www.jaybirdquilts.com/2011/02...rt-2-bias.html (2 methods shown)