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 tried a few things out to do with this a while back.  I have a feeling it depends on the scene to be honest.  I set one up, did the prepass stuff and then started messing around.  After a while of moving lights/props etc.  I noticed that the test renders had some really weird shadow artefacts so I recalculated the indirect lighting and they disappeared.  If I do much to the lighting, I'll usually get it to recalculate the lighting though - seems to me that would definately throw the map (or whatever it is) out of whack

 I suggest you leave this button unchecked unless you do animations. You are correct, it is not 'smart' and can get you in more trouble if left on.

I have also seen weird artifacts when keeping this button checked and while modifying a scene. It took me a while to figure out why there were large shadow dots in my renders!

It is supposded to be used only when doing test renders. If you move objects, change their material (mostly if these materials are reflective/refractive), change render resolution, and of course camera position, then the previously computed and stored radiosity solution will be wrong.
But you can reuse it if nothing has changed in the scene as far as indirect lighting is involved, but you want to change, for example your AA settings, or the quality boost of a spectral atmo.

A bit disappointing then. The vast majority of things I change and need to check how it looks after the adjustment will require re-calculating the radiosity. And by the time I figure out if I need it on or off I might as well just leave it unchecked always. They really should implement it automatically. Surely the program can keep track if I changed lights, camera, position and set a flag to ensure radiosity is recalculated (or at least send the user a warning). Shame, what a waste of a good idea :(

 Not really a waste, as it's there primarily for animations, not still renders.

 I really dont understand that. Dont things change when you do animations? objects move, camera moves, lights may change ? so how would it works between animation frames but not if I change it form one version of a still to another?

 Typically when doing walk-thru animations, nothing but the camera changes. Of course reflections change a bunch, but then they aren't part of the pre-render calculation. As long as the lights don't change, nor the material, nor any objects, the single uprfont calcualtion can be used to render the whole animation-- and very quickly I might add.