Where I've worked in the past I've always found it useful to indicate in a drafting manual or other internal publication exactly which of the available methods for dimensioning slots, holes, etc., is preferred. It's not that the other methods are incorrect (well, certainly some are), but rather, it keeps drafting teams (and designers and engineers who chip in) on the same page and producing drawings that look consistent. That being said, I tend to gravitate toward dimensioning scheme's that lend to achievable validation as long as design intent is clear... Harder to measure from the center of a slot than it is the two tangencies of the radii [method (c)].
