I think the most effective approach is somewhere in the middle -- you have to praise successful efforts. The more crucial part is organizing the curriculum so that every student has enough opportunities to "win" that they maintain confidence, but "lose" often enough that they don't become complacent.

That's hard enough to do in a 1:1 setting, let alone a 30:1 classroom. I think there's some benefit to 1:1 tutoring/individual enrichment any time a student isn't smack dab in the middle of the bell curve.