Help with fuel pressure relief

I'm trying to change the head gaskets in my wife's old 1993 Sable. The Haynes instructions say to relieve the fuel pressure by 1) hitting the fuel cut off switch, 2) removing the gas cap, and 3) running the engine until it dies. I did 1 and 2, but I couldn't do 3 since I already removed the intake manifold. How important is it to relieve the fuel pressure? What can I do instead of running the engine? Any help you can provide will be appreciated?

Thanks,,

E Steve K