Most days when editing, I’ll see one manuscript with the s-less version of this word and another with the s. Sometimes the manuscripts even come from writers who live in the same city. So which is right?

Either word is considered grammatically correct, according to the powers that be. Toward is more common in American and Canadian English and towards in British English, however.

Given this, I usually edit towards to read toward among my North American clients.

The exception: When a British character appears in dialogue, he says towards. This is very subtle wording that helps distinguish the character as British (while use of telly and lorry and bloody are less subtle and almost cliché).