It'll make the spray more effective if it rains shortly after you spray. Certain disease problems, especially peach leaf curl , will get worse year after year, especially on older trees and if we repeatedly have wet springs. If your trees are young you can get by without spraying for a few years. Dormant sprays will help prevent diseases on apple trees. The dormant oil is very low toxicity.
Copper sulfate is approved for use by certified organic growers.