I'm with everyone else--I never would have noticed the tree if I weren't specifically looking for it. Good save! I cover things all the time. I use all the methods mentioned above, depending on the photo and what I need to cover. I also crop close, leaving me as little of the unwanted thing as possible to deal with. I'm more likely to take the time to do big edits on a photo that's special in some way and will be used in more than just 1 scrapbook layout.
My best (worst?) examples aren't in the gallery, but here are a few I found. This photo was taken during a year when all the photos of the 3 kids together looked horribly awkward. This was the best of the bunch, and I wanted to hang it on the wall so I took the time to edit out the bench in the background.
The screen door DD is holding open in the middle photo looked terrible! It was old and dingy. I could have fixed it up digitally, but it wasn't worth the time. I just covered it with the top photo and the cluster and moved on.