Warren in Lancaster, PA
For a plane iron it is nice to have a small amount of curvature to the stone to correspond with the amount of camber on a smoothing plane or a jointer. However, since we sharpen other irons with more curvature and chisels as well, the stone is often more hollow across the width than optimum, so we wear the center of the iron on the edge of the stone to manage both the edge and the stone.

Sharpening small chisels and carving tools is rough on a stone's flatness, so it is nice to use a separate stone or at least the other side of the stone for these tools.

Having a stone hollow in length is not so critical as in width, but we usually sharpen with the edge not quite normal to the long axis of the stone, maybe 20 or 30 degrees skewed to the side. We do this because it is more stable (effectively has a larger base in the direction of travel). So if the stone is quite hollow in length, it shows up as giving more camber to the iron.

For a chisel especially it works best to have a flat bevel; a concave bevel is harder to sharpen with precision and does not perform as well.