Yeah, I do re-use them. My trees are all multiple copies of the same sprite (when they're the same tree).
For the backgrounds though, I wanted a tileable image, and I wanted a parallax effect. I went with 2048 image sizes with three trees in a row to avoid repetition being too painful. The exact same tree again and again would have been awful.
It would probably be better to place individual trees on my background layers, but this way saves time. I'll see how it goes and what feedback I get when I upload something playable. I'm more than happy to change that approach if it would make a significant difference.
I am planning on shaking up the background with 'hotspots' though. A meadow is already integrated (though it's gone completely out of whack in the .CAP I uploaded when I moved the background positions around at a later point), and I'm working on a river and waterfall system at the moment. The aim is to build them in from the background, on various parallax layers, all the way to the foreground, for a real sense of depth.