This is a problem I ran into with Centurion yesterday when we played a real-life game.
I had an unwounded 
Saruman, Rabble-rouser and 
Hillman Horde and Centurion had a 
Legolas, Greenleafand 
Arwen, Staunch Defender as well as a typical movie block Frodo. During the archery he decided to use 
Greenleaf and exert him twice. Now, here's the problem. He thought he got to assign the wounds, 
ie- kill 
Hillman Horde and stop my assault. I thought that I got to assign the wounds; I was going to give them 
to Saruman so that my 
Hillman Horde could overwhelm either Arwen or Legolas. A few games earlier we had run
 into the same problem and I had given in to preserve peace and prevent contention. Now he claims its a 'house rule' 
because I let it slip once, regardless of the real ruling and claiming that 'everyone else he knows plays it his way.' 
So, who's right?