I am a hobby builder, and a retired carpenter, so this applies to people making lures one at a time, not for production work.
I build my hard baits with PVC trimboard.
I start with a rectangular blank, trace the profile, mark the hardware locations and bill slot, and then cut that out on the band saw.
I smooth the profile with an oscillating belt sander with an 80 grit belt, and then I mark a centerline all the way around the cut out blank.
I remark the hardware and line tie hole locations, and use an awl to make dents that will still be there after I do my shaping.
Then I go back to the belt sander, and rough shape the lure, starting first with the body shape from nose to tail, on each side. I get it close, and then I begin final shaping.
I round over the bait on the sander, using the centerline as my gauge for symmetry, and I keep rounding until the shape looks right to me. I sight down the bait from the back or front to see if my curves are equal, more or less. It's pretty easy to see when one side isn't as round as the other.
Once I've got it close, I use a vibrating palm sander with 100 grit paper to finish sand/shape the lure.
My baits swim and catch fish, and it's certainly not because of my painting skills!
There is something the human eye notices when we look at anything, and that is symmetry.
If you look at what makes someone's face attractive, the first thing your eye picks up is if it's symmetrical. It's unconscious, but it's there. It's call "classic beauty".
So trust your eye when you're shaping a lure. You don't have to be exact, as long as you're close, and your eye will tell you, because it will look right.