I'm not a huge fan of the current trends of machine quilting, I prefer a puffier poly batt and less quilting than is currently popular.
I can say, however, that I do believe in no less than 3-4", or that fist size should be used in most cases.
When I go back to my earlier quilts the seams and such are still lovely but because I was quilting more like 6"+ apart, the centers of the larger pieces of fabric have mostly rotted away. So a lot of it depends on how long you want the quilt to last. 10 years of heavy use at 6" or 20 years at 3"...