Answer by Borbus for How do I save multiple buffers (of my choosing) at the same time in Emacs?

The answer to the question in the title is to pass an argument to save-buffers-kill-emacs (or -kill-terminal), ie. use the key combo C-u C-x C-c which will silently save all buffers and exit (or C-u C-x s to just silently save all buffers).