I once had a sign above my desk; "A clean desk is a sign of a misguided career."
Any time I have tried to keep a clean desk I can't find anything. Let it me and I know where to find things. Basically I work best with an archeological filing system - the further back when I last worked on it the deeper in the pile it is. I do keep a "common work" area free and always available though.
Keeping a clean desk just to have a clean desk is a non-value-add activity, (IMHO).
However, if a desk gets messy enough to inhibit design or other activities then the desk/work area should be cleaned up...
I think a lot of it has to do with filing systems and accessibility. A big flat space where nothing is more than a layer deep is a pretty time efficient filing method. However, it's not very space efficient; hence the messy desk.
I'm guessing the desk will take a two week path from clean to messy, with the time increasing or decreasing based on the number and size of tools and parts needed during that time period.
Tangible items help inspire. The kinds of eclectic things inhabiting my desk right now. A hermetically sealed
6.3V 400Hz filament transformer. A 400 Hz synchro. An ohmite ceramic rheostat. A handful of proto boards. An AFL27005S thick film hybrid DC/DC converter (Lambda Adv Analog now IR). A plasma ball. A disassembled GPS navigator awaiting a fix. A couple of stud mount diodes (400V, 250A). Plastic box with miscellaneous resistors and capacitors. Some ferrite cores of various sizes and materials. A couple reels of SM caps. A 150 amp current shunt. A Red Lion Controls counter.
And that's just what I can see. :-)