When baking with "selected to active", Cycles fails to detect when it's hit a back face and needs to flip the normal. This is important since it's the nature of baking low-poly objects from high-poly ones that polygon intersections happen, which leads to flipped normals in the final map. Offsetting the cage vertices along their normals (the "cage" option") to avoid intersections gives visual errors on fine concave features like mouth corners, noses, etc.
I wrote an OSL shader to bake out normals maps with properly flipped normals. Here is a comparison of a normal map baked with a tweaked cage offset (top) and the shader (bottom).
On a related note, it might be a good idea to flip all baked normals greater than 90 degrees to the original surface normal for tangent space baking.