I include behind the Neck presses from time to time. Heard from PL's that it helps greatly with their benches. I see this exercise done in gyms, but not by many. Older guy's who look like they know what their doing mostly.
I will usually use a bar only for a stretch and warmup. Some space their grip out too wide which may cause extra stress on the shoulder girdle (inserts/joint). A little wider than shoulder width works well for most. But make no mistake the BNP a excellent shoulder movement,with the potential of hitting all three heads of the delts. Suggest that the upper body be kept at a straight up position. If using a shoulder press bench, might make sure it's a straight up mount.
I also include pullups behind the neck, touching the nap of the neck each rep.. To me an excellent movement for stretch and strength..
Have done quite a bit of SLDL's over the years.. But find that RDL hit the lower back and hams much better, on me anyway. Will tend more to keep the whole back straight, while you may tend to round the back more when doing regular SLDL's.
Good Luck.