In my opinion, the proper way to tune is the way that is best for your song as well as the performers. It creates a cascading effect for everything in the song including the musicians.
Some singers do a better job when they sing something a half a step down. This makes the band tune a half a step down from 440. This makes guitar and bass strings have less tension which can make these instruments easier to play as well as allow a player with weaker fingers enjoy bending strings a bit easier. This can also make a song sound a little darker. The lower something is tuned, the darker it can sound.
The higher something is tuned, the more poppy it can sound. So in my opinion, there is no proper way other than the way that best fits your personal situation. I personally like 440 the best so that's what I use for everything.
-Danny