Power washing is a service that performed any time of the year. There really is no ideal time to do it as its more complete when needed. You will find that after more humid and moist times of the year will create more mold issues on shaded parts of a house. When power washing be mindful of a few things. Watch what chemicals you use. Even though the odds of damaging your house with something made for a power washer you might cause damage to your landscaping. Chemicals made for cleaning your house do not mean they are good for the plants at the base of the house. They can be toxic and cause irreversible damage. Usually the only solution is to replace the plants as they take in so much of the chemical the whole plant will not survive.
A few other precautions to take is on the home its self. Before we begin any of this work you should remove any screen covering windows. This should be done if you choose to do the work your self also. Avoid spraying your roof as it can damage shingles. Also place towels at the both of any doors that have access to the exterior of the home. Water at the pressure it is pushed out can find its way into the home and cause interior damage.