It's in good practice to use layers, especially for backgrounds. You might end up wanting to parallax the background. Can't be done if on same layer as Character sprites.
my layer structure
Gui is self explanatory
Foreground I use for grass infront of player for example
Platforms is only for (invisible) platforms not the actual drawn platform
Entity is everything else Sprites (Main Layer) Z layer used alot
Background Parallax sense of depth