If you're going to make a lot of baits, and want to do it fast, a few years back Hazmail (Pete) posted pictures of his rounding wheels. They are plywood discs mounted to a motor shaft, like a grinding wheel, but with a concave edge cut into the perimeter that he lined with sand paper. So he had a consistent curve he worked with.
If you finish sand the profile of your bait and mark a centerline, you can do as Ben suggested and sand until the line is gone, and you'll have a symmetrical bait, or pretty darn close.