Is there a difference in mixing a track so that it peaks at -12dB rather than mixing it so it peaks at 0dB and then pulling the master down -12dB?

Answer 1

If the absolute loudest peak in the track doesn't go over 0dB (not even 0.1 dB over) when you bounce the mix and there is no distortion then the mastering engineer would be able to bring the volume down to a more workable level. However, if you go over 0dB and/or get unwanted distortion from clipping the master bus on the mix... then lowering the bounce will still contain distortion.

Most master engineers would recommend -3 to -6 dB of headroom though.