I just bought a 2008 Honda Odyssey with 156,000 miles on it. The timing belt has never been replaced. Should I routinely replace it?

Yes most definitely, it is way over due to be changed out. You would want to get at this ASAP. And yes, you would want to change everything out with this amount of miles on it, water pump, idler pulley, belt tensioner, timing belt, drive gear. They make a kit with everything in it for around $260. You should probably do a coolant flush and change the thermostat also once you get into it, or have a shop do this also. And the serpentine belt and pulleys need to be checked to see if they have ever ben replaced. If you take care of all of this then you wouldn't have any problems.