Baking it on the bottom rack ensures a nicely cooked bottom crust that holds up well, with minimal over-cooking of the top. For this pie, I use a higher temp to set the crust so it doesn't slump, and then reduced it to make sure that it doesn't over-brown before the starches in the filling gelatinize and the apples become tender. If for any reason your crust is becoming too brown around the edges, shield them with a bit of aluminum foil or use a pie shield.