Another point is that some songs (ballads, etc.) are meant to sound slightly softer than more uptempo ones. Best to adjust by ear, as has been pointed out. You don't want a large perceived change from song to song, but some dynamic variance.
I have to agree with that. Back in the very early days of rock & roll (when I was growing up) the technology didn't allow for perfect recording and multiple punch-ins, etc. Just look at Chess and Sun studio recordings from the 50's. A couple of mics and record everyone in one room from start to...