As a web designer (former). I'm a big proponent of consistency. If the background of the website is white, I try and always use white background to my pen photography. But my biggest issue, up until a couple months ago, was getting white balance correct. So I resorted to grey. Now that I have learned much more about my DSLR I am gonna switch all my photos to white.
The only exception to this may be my lighter pens that sometimes look much better on black. The only issue with black is dust. It is near impossible to get a clean photo. Which then involves a lot of post editing. Novus 1 plastic polish helps too.