For me I will more likely notice the bg when playing on multiplayer, with more relaxed attitude. I mean that I've noticed that I haven't taken a note about the textured bgs as often in decent single-player episodes for example, because if the storyline/gameplay something is very interesting, my eye only watches the sprite and foreground.

Edit: After reading the first post for a second time I became a little unsure that what did you mean with noticing the background exactly. If it was either, that when do I "forget" to look at the BG, or how confusing is the background etc.