An egg wash will give your pie crust a glossy finish, while a cream wash will give your pie crust more of a semigolden, mattelike finish. Skipping a wash altogether can leave your crust looking stark.
The general rule of thumb for egg wash is 1 well-beaten egg plus 1 to 2 tablespoons of water (note: more water will lighten the golden color, or for a shinier pie, you can substitute the water with cream or milk).
For a cream wash, use heavy cream or half-and-half. No matter which you chose, a wash should always be applied just before the pie goes into the oven.