I've played with gamers that I couldn't trust. Die cheats, charsheet cheats, people who base a character entirely on how powerful they will be. I don't play with those people any more. I trust the people I play with.
Hmm...trust is an odd thing. I'd trust all of my players with my life, but there's one that REALLY wants to play an assassin. Even though we're playing your standard 'we are the heroes, we roll for initiative' campaign. To keep him happy, I might start an evil campaign some time, and come to think of it, this is probably one of those REAL tests.
If my players can create Evil characters who don't just kill everything, and have a motivation for being evil, then I would say I fully trust them.
Well, depends. If we trusted the DM not to slaughter us if we went a little soft on power, then I don't think our group would have a problem with not min/maxing. If, however, there was an abnormally high death rate, then we as a group are excellent at powermunching characters. We had one instance where we had... I think 12 deaths before we hit level 10. Needless to say, the party was munched to the gills when we turned the corner into the double digit levels.