Grease, When you measure your ERD with two spokes, measure it in several different places including where the wheel was joined together.( if this applies ) You might not see this in a motor cycle wheel but a less expensive bicycle wheel you can see where they joined it together. This spot is the toughest part to verticaly true and if your spokes are to long this is the one that will screw you.
Be very particular when vertically truing as this determines the concentricity of your wheel, hub and sprocket, no matter if its a rag joint or hub adapter. The better you get it the smoother the transmission from your engine making it more efficient, less friction and just plain FASTER!