Yes, from spine to edge. It’s a real pain to sand that way. Much harder than parallel to the spine, because of the short strokes. But I don’t know another way to consistently get an even finish. And it’s much faster than sanding all the way up to 800 grit and then realizing that there are still 300 grit scratches left. (Or more accurately, not realizing that there are 300 grit scratches left, and just being pissed off that your finish looks bad.)
Btw, I don’t do just heel to tip except at the final stage. (See also the comments above about sanding to 800, then dropping to 600 for a uniform scratch pattern.) It’s slow to only go in one direction, vs heel to tip to heel to tip to heel, and it’s not worth it unless you’re at the finishing grit.