I think orcs should be as intelligent as other races but they should act dumb to trick people into lowering their guard. So for example, rich merchants would hire orcs as body guards for secret trade negotiations thinking that the orcs would be too dumb to take advantage of that kind of sensitive information but then the orcs would use that insider knowledge to make smart investments.
But isn't it somehow unrealistic ? Entire race that decided that they should act dumb so they can fool other races, I like idea that they are inteligent but are consdier dumber, but rather then making them act I would make them race of army guys, not a bunch of savages, but rather their whole culture center around cult of army something like Prussia during reign of Frederick William I
. Therefore typical orc would easily accept orders, and woudn't think to much about because it was order, and because of this they would be consider slightly dumber culture.