Not as true with the basser tones of a V-twin. Bass is omnidirectional and you can hear a loud one coming or going from any direction. In fact, they can be hard to ID which direction they're coming from.
Higher pitched tones, like those from an in-line 4, are much more directional and easier to ID direction, but if the pipe is pointed away from you, it is less likely to be heard.
Regardless of whether loud pipes save lives or not, there are be ways to ride safely than counting on that to save your hide.