Common Sense would indicate to clean them.
As the angle of the dangle increases, performance will drop off faster. Solar panels are rated for full output at 1000 w/m2. And you can actually get more w/m2 than this on a nice sunny day. So if you do a test on a nice sunny day, you may have 1100 w/m2 to burn through the crud on your panels resulting in the rated output being achieved, and after you clean the panels, you will still be getting about the same output. But as the angle of the sun increase and you still have the crud on your panels, their performance will drop away faster.
Maybe do your dirty/clean test at 4pm and see if there's any improvement.
Well, I could be wrong again as usual, (oh that's right "Useless answer") but I'll be cleaning my panels.
