Bebi is on target if you are show casing a specific item. It should be the main focus, all other elements should be complimentary, and it should show as much of it as possible, even blending multiple shots in to one, to show extra capabilities.
As for a ScreenShot..
When I stumble on an item I like, I will find elements that match it's style/look. (not necessairily its color as I can edit that ) Example: I find a WB with smooth silver edges and carbon fiber style mesh in the center, I will search the WC libraries and the internet to find items with a strikingly simular look.
Once I have chosen an item, for example a wallpaper to go with the silver/carbon WB. I will then hue adjust the wall using the color picker from DX to as closely match the WB as possible. If it looks good I move to the next element, such as a Dock. Each time testing colors and rejecting any item that dont look like it was made to go with the rest of the theme. Some times it takes weeks to get just the look im striving for, often changing items as I spot one that fits even better.
While I like default matching themes, to me it is more fun to mix and match to create my own personal customized desktop.
To me Customization is about being different, coming up with skin/theme blends others would have not thought of, and having a blast doing it.
The most important Advise I can give.... just have fun, and If you like how it looks.. thats all that really matters. 
Here is a small tut I wrote to give an Idea of how I assemble a Screen shot.
https://forums.wincustomize.com/327514
