On my 40% Giles 300, I found a noticeable improvement in harriers and elevators by using the spoilers. Basically, it eliminated the slight wing rocking it had, and give it a perfectly stable stance laterally. The giles would stake a steep descent profile with no rock at all, and the harrier was just as stable.

A good friend of mine has a 35% Radiocraft Extra which had excessive wing rock in elevators. He added spoilers which had an impact, but it did not eliminate it. Best of my knowledge, he never did get that airplane to perform stable harriers or elevators, and it always had a pronounced wing rock.

The 40% Extra 330s I have does not seem to be affected either way. It has a slight wing rocking, and using spoilers, or reflexing the ailerons, does not change it. So I guess, is just a matter of experimentation to see if it helps, does nothing, or is a determent. Each plane is somewhat unique, and its all dependent on CG placement, airplane design, and setup.