Brake discs don't warp per-se, they suffer from DTV - Disc Thickness Variation - which causes judder to be felt.
Often caused by sloppy fitment by young garage employees. Discs should be fitted and checked for runout before fully reassembled.
I've seen garages coat the hub flange area with copperslip grease - that is a big no no!! That area needs to be spotlessly clean.
I have an on-car lathe to true up any discs that aren't running perfectly true. Skimming the discs off the car (which happens in 99% of cases) is no good.