Cold call just about never imo. Against a table maniac it can be a tool to allow him to hang himself. If there's already 2-3 cold callers in the pot and you have position and a hand that plays well against larger fields (pocket pairs, suited connectors, etc), or you can end the betting round with a call and the rest of the above applies, then it can be profitable. Yet still, good rule of thumb is don't.
Cold call % is one of the stats I look at for a fish/calling station (especially in limit games). If you see a player that is 15% cold calling hands, label him as money at the table.