At 178,000 miles you defintely need new plugs, but both platinum and iridium have a very long life. If your curent plugs are worn, you will get slightly better gas mileage with new ones, any new ones. A friend of mine drove his 1997 Taurus untill the “service engine soon” light came on; his plugs were well worn and the car was not meeting emission standards.
On our Nissan we installed iridium plugs at about 90,000 miles and will likely never replace them again.