If you catch it soon enough the detailing sprays will take it off. It does depend on the type of sap. My neighbors tree "rains" sap. At first glance is looks like tiny dew drops.

I never wash my car. Use a California duster and detailer daily. This method has worked great for me here in SoCal where we get so little rain. It doesn't hurt to keep the car garaged when not in use either.