having clean paint is essential in having a great looking car. The method that i have always to subscribed to is after washing and drying your vehicle, take a plastic baggie, put your hand in it and lightly feel the surface of your paint. If you can feel any grit then you should clay. Ideally your paint should be butter smooth.
Clay is the best way to remove contaminants from your paint, it is better than using an abrasive polish because you will not remove any precious clear coat.
For more information on claying your paint, read this:
Claying your vehicles finish