There are about 7 basic techniques in blacksmithing each of which might be accomplished by different means using a variety of way according to the time period you are staying within. Long subject. Tapering is one of the techniques.
When tapering, no matter what stock you are starting with, you should first square (or rectangle) the piece before tapering. Why? Because that gives you corners which are distinct lines to see. Taper it square, then at the right heat (low red) bump the corners in to the necessary degree whether it's an octagon or to round. Round needs to be at medium heat.
The heat and the different levels of heat plays a part as important as the hammer or any other factors.