Solar absorption is a critical factor in getting your solar panels working efficiently and producing lots of useful electricity. The last thing they need is to be caked in dirt or dust, but how much cleaning do they really need?
The YouGen website has a useful article on the subject - Do I need to clean my solar panels to maintain efficiency? - where the author Chris Rudge advises that climates featuring frequent amounts of rain (ie, the UK) should suit self-cleaning PV panels just fine. He also states that "A few of our customers have felt the need to wash their modules this summer, and have reported hardly any change at all in generation after the process". So you may want to clean them from time to time for your own peace of mind, but it probably won't make any difference to the absorption efficiency of the solar cells.
Sanyo make high performance solar panels, and their advice is:
Some maintenance is recommended to maintain optimal output performance of the solar modules. If the module surface becomes dirty, it may reduce output power. It is recommended to clean the surface of the module with water and a soft cloth or sponge, twice or more per year. A mild non-abrasive detergent may be applied for persistent dirt.
This is quite nondescript advice, and doesn't indicate how 'dirty' the modules need to be before a clean is required.