I guess it would depend on how worn it is and how perfect you want it to be. Also cost. I suggested a machine shop to true it because they can level the stone on a surface grinder so that it is perfectly flat, then do some light passes with a diamond wheel to true the top surface. It would then be flipped over and the process repeated. The stone would be squared like new again, the same process I'm guessing they use to square brand new ones.
I brought it up because I used to do this type of machine work. Having done it for as long as I did I'm not sure if I would trust any other method when you could just take your stone up to a decent shop to have them do it for next to nothing. You have to figure how often do you need this done?
I'm not dis-ing the other methods at all. Just that they could take a lot of time or in the case of the DMT money. Just another suggestion in the list of options that would work.

-Brian