OFTEN!
Our friend's son in CO had AFA as 2nd and USNA 1st, for the same MOC a girl got USNA as 1 and AFA as 2. Our friends called the MOC and they switched. SHe is a 3 dig at AFA and he is a dig at USNA.
Very competitive states MOCs look at the list, they want the best so they may take somebody who is applying for every SA and give only one nom to a specific SA.
Even in non compete areas you can't bet on getting noms to every SA...this brings us back to telling the MOC you have an LOA