It really depends on what quilting pattern you're doing. Although I was taught to start in the center, I've learned that if the quilt sandwich is basted smoothly with lots of pins, it really doesn't matter where you start. If you're meandering or stippling, it just makes sense to start at the top right or left corner. If you're quilting a motif in each block, then the center might be the best place to start. I often quilt in the ditch around blocks, then quilt inside each block.