The only real reasons for spraying are: personal taste for the glossy look and to prevent fading of images. Assuming a disc is kept in a closed black case, fading will be pretty minimal. Smudging of the printed surface is another common complaint, but let's face it, you're not supposed to be handling discs that way.
BUT, there's no reason to be using glossy-coated discs if you're going to spray anyhow. Waste of money.
My personal choice would be not to spray any discs except maybe some CD's that get used a lot.