As pictured in step 5 of the tri-fold "Welcome to your iMac" pamphlet, the cables should be routed through the large gaps at the bottom of the I/O door. The large hole provides the only convenient means of opening the I/O door. Routing cables through the hole may also cause them to become disconnected when the I/O door is opened.
The iMac (Slot Loading) does not have a door over the I/O ports so this is not an issue.