The 737-6/7/800 AFM (Doc D631A001) states in Section 1 Limitations:
FLAPS 15 LANDING
Flaps 15 normal landings are prohibited.
Your AFM may be different, but best you check your FCOM Vol 1 (Limitations - Flight Controls) or with your fleet office before electing to do a flap 15 landing unless you have a non-normal which calls for it.