you dont have to if they are working just fine but it is recommended that you do change it and about pulley or tensioner failure, i havent heard anything about it.Thanks for the replies.
I wasn't really looking for peace of mind answers. Of courses one can change everything down to the wheel bolts.
My question really is, does the pulley and tensioner need to be changed if they feel and look OK at the time of belt changing. My understanding if the pulley bearings are quiet and smooth and the tensioner feels OK then they will last until the next belt change ..... but I could be wrong.
Has anyone ever reported an Idler pulley or tensioner failure following a belt change ?