We clean any solar panels.  Typical residential installations are averaging 30K - 40K which means it is important to maintain as much efficiency as possible to make your investment worthwhile.  When dust and debris, water spotting/streaking, fallen leaves and bird droppings are considered it is not unusual to see annual buildups reducing efficiency by 15 - 40%.  When considered against the cost of the installation this drop in output translates into loss of efficiency.  This is easily offset by the reasonable cost of cleaning the panels. 

Studies prove efficiency loss of solar panels due to soiling, such as sand, dust and other dirt. The efficiency lost is due to less light coming through to the solar cells.

You cannot keep your car windshield clean by hosing it off. You must clean it in order to see through it efficiently. The same applies to your solar panels.