I was gonna do it with a cut off wheel and after I learned about the pipe cutter figured that would be alot easier to keep it straight. I think it would be extremely hard to get it back perfect square using a grinder. I used a hand file and removed a little metal to get it perfectly squared but it was pretty darn close with the pipe cutter.
Well, my chop saw has always made nice square cuts, so I figured what the hell. Took a few to set it up, get everthing level and plumb. But between a grinder with cutoff wheel or the tube cutter, definitely tube cutter.
The jig, will true the tube anyways, whatever someone uses. Within reason........