I used to do a session per song but for the last couple of years I've moved to just one session for all songs. It's more consistent and it also helps you keep a larger view of things, you hear how the whole thing is coming along, not just the separate songs. Another good thing is that during mixing you can constantly switch songs, otherwise when you listen to the same song or in fact the same 20-30 seconds of a song you can go insane or lose objectiveness really fast. Of course you have to make a lot of backups; be careful when manipulating settings/markers/etc. that are shared for all songs; have cpu and ram in mind and if need be export/freeze/bounce some tracks or virtual instruments and also the drums once you are done editing since all those cuts and crossfades tend to make everything slow.