I think it depends on the situation. if it's a big complicated piece than a template is a way to go. If it's just a small patch you cut it out, file the edges, then use a sharpie to trace it. Thing is, not all patches are gonna come out nicely in one piece.
So that brings me to another option...just laying metal on top and scribing around the metal. This way always leaves a perfect fit and worked well for me the last time I used it, which was on a 55 rocker. The shape would be a little bit of work to do yourself so I just laid the piece on top, used a forgiving and soft hammer face to shape it into the old, then scribed and cut it out. it was a piece of cake. Those are my three main go to's. It makes it easy, but yeah, a template works well.