Hand planing a guitar top to thickness is like flattening and thicknessing any board. You go from large scale to small scale irregularities. So first you use a longer plane to get the high spots into the same plane, and then you use a shorter plane to bring those high spots down. That's why the smaller sizes are called "smoothing" planes.
A No. 7 jointing plane has no use thinning a guitar top. A No. 4 smoother can do the whole job. When you have a warped board, it's best to start by going across the grain. That should not be needed on an already flat guitar top. But each piece of wood responds best to slightly different planing. I would tend to go at a slight angle to the grain, and even if going parallel to the grain, angle the plane a little to get a shearing cut. Certainly if there is noticeable runout, you need to be planing more on a diagonal, as Bruce said.
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