I automatically sack any soldier with bravery less than 40. Otherwise, they're guaranteed to freak out eventually.
Once psionics come in, I let anyone with less than 40 go, unless they are high level. Then I just leave them behind on missions where I know ethereals will be present. And I give them only smoke grenades and stun launchers. That way, if they get controlled, they're harmless. I never give blaster launchers to anyone with psi strength less than 60. I don't consider rookies as disposable, but they do get to be point men and scouts. I don't expose them deliberately but they are in the front lines. I only keep rookies with at least 40 bravery and 50 reaction. Other stats aren't really important until psionics. I screen a lot of recruits to find good ones, so I can't afford to waste any needlessly. I play very cautiously, but even so, on terror missions, alien base assaults and especially battleships, I usually lose a couple. That's Xcom.