Agreed that doing a complete backup image may not be very useful, particulary if there's a lot of junk (a lot uninvited) on the computer.
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 without having to re-load all the separate components some of which are a pain due to having to contact companies to unlock applications that I have purchased (that's a real pain).