If an employee has been working for a US company for a few years (like on a TN or L-1A visa), is it common or expected that the employer might cover the costs i

If an employee has been working for a US company for a few years (like on a TN or L-1A visa), is it common or expected that the employer might cover the costs involved in getting permanent residency? Or is typically on the employee to sort that out themselves?