I've never seen any numbers on just how "out" the frames were, but in blogs about rebuilding old race cars such as Mallocks (you knew I'd get that in there, right?); various models of Lotus racers-7, 11, 19; TVR, Triumphs, etc, etc, they commonly send the frame to somebody to be "straightened". These were not "wrecked" vintage racers, just vintage racers. I'd guess that tweaking of the frames during normal usage is not uncommon.
Hmmmm, maybe I can take my own chassis to be straightened prior
to setting up the suspension and before it hits the road.
I bet it would cost a fortune though.
That said, there's no reason not to do the best you can to level/flatten your build surface. Also, my experience with MDF was that it starts out nice and flat, but moisture attacks it very quickly. It won't stay level long if there's much humidity in the environment.
Thanks for the encouragement, JD. I used MDF for the mock-up, but decided on ply for the real chassis for the reasons you just mentioned. I live near the ocean and we do get moisture and fog on a regular basis. Even though I built indoors, the MDF definitely suffered over time and sagged where it wasn't directly supported. My ply isn't marine grade, but all the lamination glues they use now days will stand up to moisture. It seemed like a better bet.