How are you focusing in the first place? If you use a grain focuser, why don't you test it and see if it is within error margins? I personally test on a sheet of paper, but considering that I test at f/3.5 and expose at f/16, I doubt the thickness of the paper is significant. Plus, the paper often "bubbles" up at least a little bit so it's really probably several thicknesses higher than the grain focuser. Still, one less variable.