Should pensions to government employees be guaranteed?

Many cities and states are running into fiscal trouble and are reneging on promises to pay pensions to retired government employees, such as policemen. Should those promised payments be guaranteed, even if that means raising taxes or cutting back elsewhere? After all, those payments are part of a contract made between the employer and the employee. Or if money is tight for the city/state government, should the retirees have to share the same risk of default as anyone else the government owes money to?