Flat bar isn't flat, plate is close and tooling plate is flat!
I've had 12"x0.5" flat bar that's been out by 1-2mm across the width, so presumably it warps during as it solidifies. This makes flat bar a bit useless for mounting linear bearings on unless you can first machine the surfaces which the rails will mount to. Although you have to be careful since machining just one side can relieve the internal stresses, causing the material to warp further. Plate is much better, maybe a few tenths of a mm (google it) and tooling plate, as jcb said, is ground on both sides so that they are as close as you'll realistically get to being flat and parallel.

There are various grades of each. One that machines easily and is readily available is 6082-T6