This is just an informal observation, but most that I have talked to would say -- all things being equal -- wear a tartan associated with your father's line. However, I know plenty of Scots who wear tartans for their mothers and grandmothers, so I don't think many see this as a hard and fast rule, rather than a general guideline to help people pick what tartan to wear.

I think most would agree, though, that if there is a tartan directly associated with your surname, that would be the primary one to wear.