In v7 there is a new option among the render options - re-use indirect light. It saves a lot of time on subsequent renders, but I'm not sure when it is OK to use it. It is fairly obvious that if you do anything to your light sources you cant tick this option. But  all the manual says about other changes is ' if significant changes have been made to the scene, the radiosity calculation may no longer be accurate.'. But what constitutes a 'significant change'? can I change the material on an object? can I  adjust the position of an object? can I move the camera? can I re-populate an eco? change render size?
Also, is there any intelligent checking within the program to catch user errors - like I left it checked but I changed a light, I  selected only part of the area to be re-rendered but didnt tick 're-use'?
I suspect it isnt intelligent at all, but it should be...

I'm hoping some of you already figured this out so I wont have to experiment that much.

Thanks for your help

"I paint that which comes from the imagination or from dreams, or from an unconscious drive. I photograph the things that I do not wish to paint, the things which already have an existence."
Man Ray, modernist painter
http://artpearl.redbubble.com/