1. An image forming system that is capable of forming images on sheets and performing post-processing on the sheets, comprising: a display device; a storage device that stores a plurality of maintenance items, and sub-maintenance items associated with respective ones of the maintenance items; a first determining device that determines whether or not a maintenance operation based on a selected one of the maintenance items has been completed; a second determining device that is responsive to a determination of said first determining device that the maintenance operation based on the selected maintenance item has been completed, for determining whether or not there is any sub-maintenance item associated with the selected maintenance item, by referring to said storage device; and a display control device that is responsive to a determination of said second determining device that there is at least one sub-maintenance item associated with the selected maintenance item, for causing said display device to display the at least one sub-maintenance item.