The B737 Flight Crew Training Manual contains the following guidance:
"Landing Gear Extended Descent
The rapid descent is normally made with the landing gear up. However, when structural integrity is in doubt and airspeed must be limited, extension of the landing gear may provide a more satisfactory rate of descent.
If the landing gear is to be used during the descent, comply with the landing gear placard speeds."
At normal cruise levels with a cruise speed generally around 250/M.77 on the NG the gear extension speed is not an issue (270/M.82), and the gear can be lowered without waiting to slow down. Upon reaching level off it could be necessary to slow down to the retraction limit of 235 knots.