I got this from RaceTech.com. I had them "arc" the rear brake on my Phaedrus projet, CB750K2. That is an old art of first turning the brake drum on a brake lathe. Then grinding the brake shoes to match the arc of the drum for maximum contact.
But to the subject of this post, anyone can center their brake plate when installing. You should tighten the axle only a bit. Then engage the brake either by hand or have someone step on the pedal. This will center the brake plate, taking up any of the slack clearance it may have between it and the axle. While the brake is engaged, tighten the axle.
If yours was off, this can improve the feel and power of the drum brake, reducing fade, and potential of glazing the shoe at a hot spot.