anything over 10" needs to be retained
(I know igge, "no ****" right?)
Anyway, you need to retain them front and rear. You will find that with the rear retained you will force the last bit of travel out of the radius arms, and the truck will be much more stable.
But don't listem to me, I haven't retained mine yet. However, I am still running those pukey ome's. If I ever re-do the shock mounts I will be welding in mounts for the spring retainers.
Ask bill morgan about the performance of my rover before and after spring retainment..