Its all what you are used to driving and want the truck to do. I have DD'ed my K5 with a welded rearend, no rear shocks, no swaybar, and only 1 shock per tire in the front. I don't have much problem with it.
Dual shocks were offered on the K5 in the HD front suspension so they would be better suited to have plows and such on the front. Diesels got the option becasue of the heavier motor. It was a purpose option along with the more shock fad that started in the 80's too.
It is not necessary and will do nothing but stiffen the onroad ride and limit flex offroad. If it was necessary every K5 would have had it. I personally would want my DD to ride better if I had the choice. Single shock will make it ride better.
I think the single shock has more pros then cons and is the better option. But it is your truck, your money, and your choice.
Also FYI if you do offroad with the single shock you have a decent possiblitity of denting and ruining the inside shock. I did with my 2.5" lift when it had the dual shocks. Dented both tubes on the spring packs under flex. That was with the swaybar on too mind you. So that was $50 worth of shocks ruined by a poor GM design.
Harley