Tip: A better way to center your guides

I ran across this tip at the GraphicGuy's Blog, and I thought it was pretty cool, but when I showed Dave Cross, he took it up a notch. The tip is how to mark the center of any document with guides. I've always done it this way: Select all, bring up Free Transform, then drag out guides until they line up with the center point. This is better: go under the View menu, choose New Guide, and type in 50% (you do it twice: once for horizontal; once for vertical). Dave noted that this won't work as an action unless you first change the Ruler units to percent (rather than inches or pixels). Then it works. Cool tip!