It's kind of hard to give advice without seeing a picture of it and more of a description about the design of the bow.
When I cut out a riser on the bandsaw, I stay about 1/8" away from my pencil line and then use a spindle sander for inside fadeout curves and a disk sander for outside curves to get to the final shap. Like James said, the riser needs to be fitted to the form or you won't get good glue joints.