Kind of depends on the glaze, glazes with lots of clay in them tend to not hardpan. Bentonite is typically used to aid in glaze suspension. At 1-2% there is enough to help suspension, but not enough to mess with the glaze.
I don't always add bentonite, but I DO always add CMC gum solution to my glazes. It helps with suspension, flow off the brush/brushability, and more importantly it forms a "hard coating" that keeps the glaze from rubbing off in greenware state.