If you want to maintain the exact airspeed, then yes, you will want to decelerate. The reason few people teach techniques for the 'into the wind' turn is that it is not nearly as critical or time sensitive. You don't want top get too involved in the "math" of why you're doing things until you have a good understanding of the techniques. The primary concern when flying in high DA, precipitous terrain, at high GW is that you maximize performance and efficiency (for the customer, that means flight time) while keeping the aircraft from hitting things (like trees, rocks, etc.)
One of the common mistakes is making a downwind turn at relatively low speed and having the aircraft build a high rate of descent as the airspeed drops and power requirement go up, without the altitude necessary or excess power available to arrest the rate or "fly out". It happens very quickly in the high wind conditions found in mountanous areas, and the aircraft is usually being flown at the limits of performance because of environmental conditions/customer requirements. In the Rocky Mountain region, the average wind speed on many days could easily equal 50-60% of your desired manuevering speed. You have to account for that by adding a little cushion (V+10,20,whatever) and by anticipating the changes in power required accurately and quickly. The other critical requirement is that you be able to read the terrain and its effect on the wind. In many areas the terrain actually dictates wind direction, velocity as much or more than the free stream/prevailing weather. One technique that I have taught is to try to visualize what effect the terrain features would have on a flowing mass of water. The atmosphere acts in exactly the same fashion.