jike,
Be consistent with your design approach in sizing the beam. If you used the reduced live load to design the beam, use the reduced live load for serviceablity. The purpose of the reduction is to account for the fact that the structure may never actually see the full live load. In other words, compare apples to apples.
If you are comfortable from a design standpoint that the beam will not fail using the reduced live load, the same should apply for the deflection.
Good luck.