OOTP's reliever system leaves a great deal to be desired. You may want to consider doing
like some people did and just clear all the bullpen spots.
I figure there's two systems most everyone would be happy with:
1) Modern MLB - set relievers for each inning given the score; best reliever gets put in the 9th in close games, 2nd best in the 8th in close games and so on
2) Classic MLB - bring your best relievers into the game duing the highest leverage situations as long as they're not tired; close game in the 7th with men on? bring your #1 stopper in now because the game might not be close come the 9th.
Ideally, you could tell the AI which method you prefer and then it would do the rest. Or if you'd rather tell it exactly what to do, something like the overall team strategy screen where you get to set things based on inning and score differential would work for both systems. The only thing is in addition to inning and score differential, you probably want to also add in the "men on base" situation.