It's always a good idea to clean the area that you are about to prime as well as possible...The cleaner you get the car, the less dirt you will get in your primer. I usually don't pressure wash a car before I prime it...I blow the whole car down followed by wiping it down with a product called "Spirit Wipe", you can use any product that is designed to remove any contaminants from the surfaces that you are about to prime...many people use Wax and Grease Remover, (I use a lot of either of these products when I'm blocking the car...I wet the panel down with a rag and while it's still wet I check the reflection for any imperfections in the body work). These products will also act as a "dust magnet" so to speak. Once the body of the car has been wiped down and is dry, tack the body. I prefer to use a blower while I'm tacking a car down, it seems to get rid of any dust missed when wiping it down.
NOW...the most important part...prime your car...LOL