If in doubt change it I would say!
Age is as big an enemy as mileage, how old is the vehicle? At work I have seen several go ping at about seven years old, because at about that age the belts are pretty perished and what happens is usually that they don't break, they just lose all the teeth along about 6" or so of the belt, and the engine stops with much the same expensive crunch as you get with a broken belt
Personally mine's getting done no later than four years, the vehicle only has 16000 miles on it (2.5 years old now) but I ain't risking it.