Any time that I need to find centers, whether it be between the vise jaws, or splitting a piece in half in the "X" position, I always have touched off, zeroed, went to the opposite side, then divided the value in half. Of course this is when having a digital readout. No matter what I make, I like to work off of center instead of one end. I find that most parts, when looking at squares and rounds are mostly symmetrical. It is not that way all of the time, but with the majority of parts, there is a center. One reason I like doing the work off of the center is that if you have a piece a tick longer than 6", then you machine one end in a 6" vise, then you go to the other end and mill that end off to clean things up. It also depends on what the part is, but for the most part, I like to work off of center.