Absolutely do not cut your scallops until after it's been quilted AND you have the binding attached. Think about trying to quilt scallops and having to make sure all those bias, curvy edges stay down and behave....
When I made my scalloped quilt, every tutorial and blog out there said to use a plate or some other round item. However, none of those were the right size and shape for what I wanted for my quilt, so I made my own scallops with the help of EQ. Because I wanted my scallops to curve around my on-point blocks, I measured how tall they were, then went into EQ and drew a block with the height and width I wanted. The scallops I wanted were 15" tall and 5" wide. I then drew a "bezier curve" in that block, printed it, cut it out, taped it to my quilt, and marked it.
HTH