If you want the building to be straight in the photograph, it has to appear straight on the ground glass. This is why lenses with movements are called perspective correction lenses in the small format world.
Originally Posted by Kevin Kehler
If you want the camera to be level, it has to be level (actually, the film plane and lens have to be plumb). Those two things are separate.
Maybe I'm missing something, but the base plate of a field camera has very little (none) impact on the image, so leveling that is only in aid of squaring off the front and rear standards.