I just use photoshop or illustrator.
You can set the units to inches instead of pixels and then if you need to you can print things out to scale.
It isn't really needed unless your cab has a lot of curves in it though. Like he said, just draw it. Also giant hunks of cardboard are your friend. On a few cabs I cut out the whole side in cardboard to scale, just to see how things are going to look.