Further to my initial post, it is instructive to see the following in the B737-200 FCTM under the sub-paragraph Normal Maneuvring and Flap Extension:
"The flaps extend mainly aft and slightly down to flap 15, increasing drag very little. Deceleration, descent and flap extension are relatively incompatable. Normally, extend the flaps on the flap/speed schedule in level flight using the minimum thrust required to maintain altitude while decelerating, and trim out the resultant pitch changes as they occur".
All of the above would suggest that Boeing recommend extending flaps from up to flap 15 in level decelerating flight - not in a descent.