I have seen them done both ways as far as well. I think it is done from the outside so that the wedge can not be pulled out. As it pulls the wedge gets tighter. I think it is done from the inside so that you can tighten the wedge after shrinkage. I have also seen it as just a straight 3x2 block (dovetail with a 2" clip and the mortise 1" bigger than it needs to be for assembly) This makes it a little easier to cut, the block still prevents the dovetail from sliding out. I prefer to do it from the outside with a large through wedge that you can pull from the inside if there is any significant shrinkage. In a tie situation gravity will do a lot keep that dovetail where it needs to be.