None. I wait to scrap until I can do a layout from start to finish. There are a couple of times that I'll come back to a layout and nudge an element this way or that, and yesterday I went back and swapped out pics on a layout because I found photos from my DSLR that turned out WAY better than the photos I had snapped with my smartphone; I forgot that I had taken my DSLR with me that day and that I had photos on both.
Other than those minor changes (swapping a pic, nudging an element), my layout is finished before I move on to anything else.
The one exception? When I'm working on an album (like the Summer Love Mini Album from Nettio), I go through and set up the entire album's bg papers, etc, and then pop in pics later. If I don't feel a background I chose was suiting my photos, I can always swap it out, but I like to do the whole album at once, even if I don't know what photos and elements I'll use, because I want it to look cohesive. So, those sit unfinished until I get to the photos for that page.
Oh! And I'll start a layout and then trash it if it isn't going how I want it to. In fact, I've trashed FINISHED layouts that I've gotten all the way done, even through the shadowing, and I think "I don't love it." I'll just trash it and do it again. No reason to have a layout I don't LOVE in an album I'm printing.