I dive with the valve closed unless I'm making an ascent, then I tend to have it ~half open and let the variable pressure dump do the work for me, I only have it fully open in the last 5m or if a serious problem occurs at depth.
I only use the BCD for surface buoyancy and to stop my tank from wandering away! The suit's great for buoyancy control, it takes more practice than a BCD, but get it right and it'll hold you in a great position in the water. While you're training I'd stick with what you're doing now, when you get more used to the suit take some time to play around (safely) with it and find the way that you like best. Like any motor skill, practice it enough and opening and closing the valve will become an instinctive action.
Welcome to the world of cold water diving!