You don't need to use 6v but it may be convenient to do so. I've fifteen plus planes in the hanger, all of them use LiFe battery technology because doing so simplifies things.

That said, I do have one old Cub flight system and a couple of ignitions with 4.8v restrictions. I've chosen to voltage drop for those requirements with diodes.

Each RC pilot has choices of what works best for them. There are no requirements that one must do things like others. If a method works for you... your good.