In my experience, it's less trouble and cost in the long run to just replace the tensioner when you replace the belt. They don't seem to last much longer than a belt would last anyway.
2005 Dodge Grand Caravan SXT, 129K mi.
Former owner of:
1988 Dodge Caravan SE, 2.5L I4 (1988-1995, 99K mi); 1995 Plymouth Grand Voyager SE "Rallye", 3.3L (1995-2004, 169K mi); 2003 Grand Caravan Sport, 3.8L, (2004-2008, 106K mi), 2008 Nissan Quest, (2008-2010, 25K mi)
2008 Lexus RX400h (Dark Gray), 112K mi; 2005 Hyundai Elantra GT (Blue), 89K mi