And one more important thing is to try different kinds of backgrounds for the skin.
Too true, some of my past mockups Ive had up to 20 different wallpapers on layers underneath that I turn on and off to check how the theme looks over different environments. This helps in developing very versatile themes.
Its also good once you have imported your graphics into SKS and applied them to change your wallpaper often.. for the same reason.... May look great over the wall you have made but not so great over many other walls.
I may change my wallpaper 75 times a day while making the larger elements of a WB.
This is especially true when testing transparency levels in frames, taskbar and start menu.