Edit Polys / Merge Verts : value of 0.01 (mine by default when mergin' after a combine)


ps : for your previous problem, the first action after combining polys is, also, a Merge Verts (i also select the central vertical line of verts in the front view if i'm not sure about the value, in case of high poly modeling e.g.)

The point is up here, if you don't merge verts, you keep the invisible duplicated edges, so the open edges.
After combining and merging, set these edges defined by the same line of verts, as being soft edges (Edit Poly / Normals / Soften-Harden). This removes the crease.

Therefore, you understand now there's no need to Soften-Harden before mergin' verts << Open edges stay "Hard"