I once skated a 2nd cut file all over the frets after spraying them with Dykem blue to make sure I wasn't taking too much off. Finished 'em up with a Stew-Mac fret file and the frets played great. Files are notorious for warpage, but I didn't see any daylight between the frets and the file before starting (that's a more precise test than might be thought) and I think if you play an average action like I do, there's a decent margin for error available. The whole reason for leveling the frets is to prevent buzz, and with a little relief and average bridge height, we can easily eliminate buzz. Yah, maybe in theory, some strings get CLOSER to buzzing on some frets than others, but a miss is as good as a mile. Anyway I had no probs. I only got the quartz polymer beam instead of using the file again because I want to think I'm going to do a better job this time, though I was totally satisfied with the last one I did.
"Do good. Fight evil. Press on." --Motto of Jay W Kelley, Lt Gen, USAF (Ret)