how do I turn on the auto-spacing-hinting functionality
 

I'm not sure what to call it. I recently opened an ex-coworker's OmniGraffle document and it had this great functionality I have never seen in any of my own documents. As I moved elements around on the artboard, OmniGraffle would 'hint' (for lack of a better word) the spacing of my element in relation to other similar elements in my chart allowing me to standardize the distance from one element or group of elements to another.
Brilliant! What is this and how do I turn it on? I looked in all the tutorials I can find online and didn't find any reference to it.

Also, can I suggest creating a hierarchy chart that works better with websites, which generally have 1-7 main and secondary pages but then potentially hundreds of tertiary pages? I have to hand-format my sitemaps to keep them from being bottom-heavy pyramids, which can get tedious.

Check the "Guides" submenu in the "Arrange" menu and make sure that the smart guides are turned on.