this is such an ironic post! You pretty much described EXACTLY how I've been doing it. I shim, and use a 3/4" whiteside bit to flatten... process that whole side, then flip and flatten the other side and process. Good to know I was on the right track, but that said... on these les pauls I had to take off 1/4" of material and it took a long time, not to mention that if you don't shim it just right... you flex the material and distort the index pins. Perhaps I would get the hang of it if I did it more but either way the drum sander will be nice to have around. I'm sure I can always get my money back out of it if I find I don't use it.
thanks for your response! I honestly wondered if folks would think I was nuts for not squaring my material prior to cnc - guess not!!