I know, this question sounds old, but there is a twist.
If you want to use image stacking to reduce noise, you have to choose a different seed for every render.
To do that automatically I found the following methods so far:
#frameas driver in the seed field. This however only works if you have multiple frames (i.e. render an animation) - not for a single frame rendered repeatedly.
- using the small Python snippet
noise.random() * 10000. This works fine for normal renders, but it screws up viewport rendering (at least in 2.71): the render never ends
Is there something, that can do both?