[quote="-Axiom-"]How am I supposed to cut a valley or rake from the back?
Blades are a cost of doing business.[/quote]
Before nailing, mark a notch on the top & bottom of your cut, flop shingle onto face, eyeball your straight line or lay a thin (straight) scrap of wood & then scribe a line along the wood.
Start to cutting.
Agreed that blades are a cost of doing business, but there’s no reason to not try & lessen it when you can.