A slight arch is good insurance, even for composites and laminates.
Many builders regard it as mandatory for solid back plates, to allow a safety margin for expansion and contraction of the plate with moisture changes.
It takes only a few minutes to pencil the desired profile onto the braces, and hand/machine sand to the line.
For very slight arching, the build is the same as a flat plate build, although you may consider tapering the linings (laterally) to ensure perfect contact of the gluing surfaces.
I have built with flat plates/braces with no adverse consequence, but now prefer to use a slight arch.