I wouldn't provide one if it is not provided for in your contract. It's funny that you should ask because every medium voltage ductbank that I've ever been involved in had a spare conduit in it. That especially holds true for primary coming from the utility.
Hit the customer up for an extra. If they ever have a failure without an extra conduit to pull in a new feeder, they may regret having not provided one while the project is under construction.
Extra Change, Bling, bling,