Re: Shellholders+Dies matching
About the only way I can think of that the person that told you that might have came to that conclusion is that sometimes, *sometimes*, you wind up with a die that won't size the case quite far enough to bump the shoulder the way it should. It's possible that by changing shellholders they might have 'solved' the problem... but generally speaking most of the shell holders are designed to plug into any of the regular presses and work with any of normal brand of dies - the dimensions are fairly standardized across the brands. Some flavors (Hornady comes to mind) may have less radius around the inner portion and hold the case more snugly side-to-side, and others might have narrower grooves where they engage the extractor rim, but the dimension from the top of the shellholder to where the case head sits is pretty standard. There may be some +/- tolerance between brands, but probably at least as much variation exists between shellholders from different production runs from the same company.
