Okay, it's been about five months and I have developed what I assume is common practice for interaction designers who use Fireworks to create mockups for multi-page or multi-view on-screen experiences (websites, computer software, mobile sites or apps, etc.). For anyone else who's curious, here are the steps I now take to manage assets across pages in Fireworks. Not news to the old-schoolers, but perhaps there's a newbie or two out there who has searched for but not found the info they want on how to organize a Fireworks file.

1. Make primary page elements each into their own symbol (Header, navigation, buttons, text fields, any common page components). Don’t hesitate to combine symbols into another symbol. For instance: I often have a search field with a button and a logo that I create into separate symbols and then combine them together (not 'group', but make a new symbol with all three symbols in it) into one symbol so that whenever I update any of the individual symbols those updates get reflected across all the pages where my combined symbols appear.
2. If those primary page elements don’t move much (x,y), put them on a master layer.
3. If you have elements that will be reused across some pages but not all pages, put them on their own layer and share them across the pages where you want them to appear.
4. When you duplicate a page, those layers which are shared will not literally duplicate (creating larger file size) but rather be shared as a new instance to the new page, but those elements which are not on a shared layer will be duplicated independently of the other page so that you can move them around and change them without changing the items that look the same on the page you duplicated from. If you do this a lot (duplicate pages that contain non-symboled, non-layer-shared elements) you will create files of very large size, and Fireworks will crash, repeatedly.

Hopefully that helps someone. Please reply to this post if you have better ideas.