You can do it anyway that you want to!
Here's the reason that many of us do the hogging with a metal tool first.
When you hog out with the thin tiles, a deep mirror will end up needing to have another tool made with fresh tiles at some point along the way and you don't want to be doing that when you are at the 25 micron point where tiles start wearing through, you can do it that way. I prefer to get the biggest part of the glass out off of the mirror before I make the tool as that will mean that I won't wear out the tiles before I get done. The front side of the tiles are already at something resembling the ROC desired so all of them will be worn about equally and most of the wear will be in getting the two surfaces matched to a curve rather than generating the curve.
Remember that if you need to drop the center of the mirror by .130" and the tiles are .125" thick then you're going to be replacing the tiles at least once and maybe twice in getting the curve generated, depending upon how hard you hog. Why spend the money on the tile several times when you need to spend it only once. BTW, the piece of metal just needs to have a circle so that it doesn't have a prefered direction in your hand or on the glass. Anything can be useed including a scrap piece of steel, brass, bronze or aluminum or so forth. Cast iron has been used mostly because it's cheap and easy to do. Other metals work well also for this purpose.