neck seams are why my alts all wear chokers or collars.
They seem to be unavoidable >_<
Light can make it better, or worse. I've noticed that even if I have no seam line, one can show up the moment I add a tattoo or a BOM layer. I suspect it is the edge of the layer/texture/tattoo showing up even if there is no texturing near that spot.
I also suspect part of the issue is the difference of the items. Hard to put in words atm so let me ramble.
Skinmaker A makes full skins.
Bodymaker A makes just bodies, no heads
Bodymaker B makes full bodies with heads attached
Bodymaker C makes full bodies but the heads are separate
Headmaker A makes only heads
Skins on BA body may have a seam, depending on how well they made the neck blend
Skins on BB body have no seam as it is all one piece
Skins on BC body probably do not have a seam as the parts were made by the same person under the same settings, maybe even as 1 piece then cut into two
Skins on HA head may have a seam depending on how they did their neck. It may hav a seam on one body but not one from another maker as it matches up better
In a way it is like the old days. Back when we had system clothes and prim/sculpt parts like sleeves/cuffs. You could use the exact same texture on the systm clothes, the prim/sculpt parts but more often than not, the prim/sculpt parts looked out of place/obvious as the textures did not blend togther as well as thy should since they all used the exact same texture.