Like anything, tying up too tight can cause damage because the blood isn't allowed to flow freely. In case of the penis, when it becomes erect, it does so because it fills with blood. There have been cases where its been tied so tight that it becomes so swollen that it needed hospital attention.
Personally I would use equipment that is more suited such as a parachute stretcher etc - things that are specifically designed for this sort of play.
If you must use cord or rope, then make sure that its tied not too tightly but also its done so in a way that it cannot accidently slip and make it tighter.
Also If this is going to be for an extended period, I would perhaps use a chastity device instead. These are not only more secure, but they are much safer and less likely to do any damage