If the mac address is not known to the switch then it is a unicast flood and if you have blocked that on a port the end device won't receive the traffic even if it is meant for it.
Once the end device communicates with the switch then it knows the mac address so it will receive traffic.
I have never used that command but my understanding is that you are relying on the fact that most end devices are fairly chatty and so the mac address is always known on the switch.
Of course you would most definitely want portfast enabled on end device ports because otherwise you could see a large number of TCNs which causes the switch to flush the entries of the mac address table of any non edge (ie. non portfast ) ports.
It really depends on what your requirements are and why you would want to block unicast flooding in the first place.
Jon
