My cars don't see a lot of mileage so the only time I replace serpentine belts is when they are cracked in the ribs look badly worn from the back or if they're stretched beyond the wear marks on the tensioner.
The only time I've replaced tensioners or idlers is when there is a lot of movement from the tensioner or the pulleys make noise or have play in them.
And I also usually thow the old belt in the trunk just in case like fan belts on older cars but I've never needed it. Of course the one time you do if it throws it or it shreds it wont be there.