I definitely agree with not moving the goal posts, I think this makes the scroll indicator useless since we make mental 'bookmarks' based on position. I believe a good compromise would be to keep the document height fixed and without loading everything. This wouldn't work for 'infinite scrolling' solutions, but scale-resetting could be minimized this way.
I'd like to propose a sixth rule: don't move the goal posts. In other words, don't load additional content dynamically. This violates the user's expectation of when they will be finished scrolling. I find this frustrating, and I doubt I'm the only one.
Thoughts?