You aren't going to like this answer

, but here goes!!
Off for music is probably best, but sometimes it's nice to have the crystaliser on when playing low quality mp3's.
For games, well, nobody can help you really, because it's all what you think is good. I don't use the SBX surround at all when I am using speakers, but if I have my headphones on I turn it on. On other forums I see people using lots of different settings for the SBX surround from 10% to 91% to 100%
And it varies from games to game as well.
So, in short, there isn't one setting of sbx surround that works better than others. Just try various settings and see which one sounds best.