From my experience it is no trouble to momentarily hold level flight to drop the flaps through to Flaps 5 then tip over into the descent. The low drag recommendation by Boeing (737-200 FCTM) is gear down and flaps 15 at GS intercept (having already set flap 5 in level flight) then descend at flap 15 manoeuvre speed until ready for landing flap.
One can rationalise as much as one likes, but I think it boils down to a lazy flying practice to push out the initial flaps while descending, rather than in level flight.