Base painting is EXTREMELY important when you go to paint a prop. Coating a prop with a base coat of paint ensures that later layers will stay on the prop and that it all paints evenly. It also smooths out the surfaces and makes sure the paint will go on more evenly. It is a good idea to coat with a base color similar to the color the prop will be. However, altering the color of the base coat will alter the color of the top coat overall.
Do note: you ALWAYS need to make a base coat for metallic paints. :)