Honestly not as long as the wallpaper is fairly plain, so that I can change (most of) the color to match that of my terminal background, and keep the transparency level low. I think it also depends somewhat on the individual looking at it, I barely even notice it when focused.

Like, this particular wallpaper is pushing it in terms of reading interference (but, I really like it). However, something like this wallpaper is perfect for transparency, I use Arch, btw.