Ditto yackeydo.
The stereo field consists of left and right channels(also front and back, but later for that). You want to fill up your stereo field as if you are listening to the song at a concert. Imagine you are in the audience of a live musical performance, lead guitar to the left, rhythm guitar to the right, piano just off center, vocals and bass typically dead center, etc. etc.
THere are no hard and fast rules.
In the end it comes down to how does it sound to your ears. If you didn't pan anything you would basically have a mono recording.
If you are talking about panning left to right during a song, as Yackeydo said this is typically done to create a specific mood or a dreaminess quality.
The next time you listen to a song listen to where the individual instruments are coming from, hard left, hard right or somewhere in between.
There is actually a pretty decent explanation of this in the Home Recording For Dummies book.
Hope this helps.