Sending a student to the principal's office should be a last resort. For instance, when a student is insubordinate and will not do anything you tell them after several attempts, they should go, especially if the student becomes rude or aggressive in any manner.
There are some other more extreme examples such as fighting, bullying, tobacco usage (something I witnessed today as I was leaving school), etc... where sending a student to the office is a no-brainer.