The discs have a minimum thickness stamped on them: for some reason the rears I've got have the figure in Imperial, 0.18"
the fronts are 4.5mm min thickness. I think you could skim them to true them up, but I'd stick to the specified figures within 10%....brakes are really a bit important....here in England the yearly Mot test will fail a bike if the tester notices: many testers don't bother :yow