This is something I have wondered about.  I generally lump each category of relief pitcher as the same, i.e., all closers are A, all set-up are A, all Long are A.  This gives Sparky the chance to use them interchangeably.  Does anyone know how Sparky makes the choice, if all pitchers of a given type have the same fatigue?  Is it therefore a random selection or is their some hierarchy in the SIM that makes the selection?

Q: How does Sparky pick a reliever? 
A: Badly.

Contrarian's post is an excellent one to read but I believe the short answer to your question is OAV.