Could you link the model you are looking at?
And, Unless you are planning to OC. I would say that it is unneeded. Actually, IMO having the retail cooler is fine if you are not going to OC either. So I would say since you are getting a 3rd party cooling fan, you will be fine without a duct.
I have a "side duct" its attached to the side of my Coolermaster CM5 case.. I have removed it before and I haven't really noticed more then 1-2C difference with it on or off.