In general, efficiency reductions stemming from dust, etc. are negligible, and regular weather patterns should be sufficient to 'self-clean' the system over time. 'Regular cleaning' is a value-added service you include in your customer proposal, or even an opportunity for recurring, monthly revenues.
However, when panels are covered by snow, etc. such that direct sunlight is obstructed, they face practical limits on energy production. You are encouraged to clear the panels with a soft roof rake, with special care not to scratch the panels.
Keep in mind, that the sunMAX Design & Sales app takes into consideration the annual production of a solar energy system, so in areas where Net-Metering is allowed, excess production during Month A will compensate for lower production in Month B (if snow occurs in Month B).