Quote: Measuring from corner to corner you can still be out of square,
The trick is to make sure you have the SAME measurement corner to corner
Quote: I'd still notch the posts and use them to support the framework any way, there will be a lot of weight on the frame using just nails to support it
The posts are flush with the tops of the 2x8 band boards. the walls are ridgid when they are plywooded. The posts will bear the weight of the structure. I'm sure Michael will be using lag bolts/screws as well.
If you're a little out of square it's not the end of the world by any means. The really important thing is that the 2 walls that carry the rollers are paralell. You need to have the same measurement at both ends of the building. Make sure the wall plates are cut the same and you'll have no worries there either.
It's also more critical that the support posts (out riggers) are straight with the top of the walls rather than level. If the building itself happens to be a fuzz out of level just keep it straight with the walls. We build on slabs often that aren't level. The roof will function perfectly.