What I did on my last houskeeping which included a complete HD format and thorough clean of the HD, was to install all the basic software I use including any specific configurations for Office etc; no google earth or that sort of program. At that point I made a system image. I figure that, in the event I need to clean up my system, I have a useful starting
This is largely the correct approach if you are going to be doing system images.
Its a similar concept to how things work when you're rolling out a few hundred desktops to employees at some large companies. IT admin create a base system image (or images) which then get rolled out to desktops (although the tools and methodology used in the corporate world is somewhat different due to the ready availability of servers for offloading some aspects).
The whole point though, as you correctly infer, is that images have to be done on day zero in a clean state.
Using system images as a form of backup throughout the lifecycle of your PC is, however, very much not the intended use. System images full of accumulated crud are a fat lot of use to anyone.