If you're having difficulty tuning the mast straight on a rig that is deck-stepped, be certain that the mast butt bears evenly on the step. If the walls of the tube are not sharing the load evenly, the mast will want to bow out of column once compression in the form of rig and sheet tension is applied. Check for any gaps around the mast butt while sailing, as you want that interface between mast and step to be locked in, particularly side to side. There is an exception on some smaller boats, where you can induce or reduce pre-bend by shimming under the mast, to purposely load either the front or back of the mast. For example, in light air a very thin shim (like an aluminum can or lucky coin) could be added under the aft edge of the step, which forces pre-bend into the bottom section of the mast by loading the aft wall more than the front. That same shim might be removed for medium air, and even added to the front of the rig to prevent pre-bend in heavy air when you are already bending the mast with backstay and boom vang.