The absolutely most basic way to build true is to start with a perfectly flat building table. Everything else follows from there.

The thing is, it really wouldn't matter if a rib is tilted a degree or two. The wing will still build fine and fly fine. I never use rib alignment triangles and all my wings come out fine. It's easy to eyeball a 90 degree to the table, zap it with Cyano, and it's done. I've build entire half-wings in an hour that way with no problems.
But if the building table is twisted in any way, then that twist will be built into the wing permanently since it's that flat reference that establishes the "trueness" of the wing itself.

The same thing goes for the fuse. If the fuse is built with a slight twist because of the table not being perfectly flat, then that twist will be built into the fuse so that the tail section might be off from the nose orientation. Even a little bit off here will affect the flying characteristics of the plane.

To get a perfectly flat building surface, I started with a large, steel top workbench such as this:
http://www.grainger.com/Grainger/items/7D179?Pid=search

After I set it up where I want it, I sight along the surface to make sure each corner is square to the others (trust me, you're eyes are good enough to do that accurately). I shim under whatever leg I need to in order to get it true, and being steel, I know it won't change from it's "true" condition.
Then I cut a piece of high quality plywood to go on top of the steel, and then I cut a piece of sheetrock to go on the ply for a surface to pin to.

Sounds like a lot of work, I know, but imagine yourself down the line after building maybe 10 planes and how you'd feel if every one of them gave you trimming problems due to twisted structures.