A useful general rule, but not always possible to implement:
* When one of the columns in the candidate key is nullable
* When there is no candidate key (no unique combination of columns)
It may sound like I'm talking out of both sides of my mouth now, but if the table in question doesn't have a VERY short half-life, those are two reasons to simply create an identity key, either as an identity integer or a GUID.
__________________________________________________
Against stupidity the gods themselves contend in vain. -- Friedrich Schiller
Stop, children, what's that sound? Everybody look what's going down. -- Stephen Stills