It's not required when you file. It might be required if you are audited, but it depends on the nature of the relationship between you and the entity you are contracting with. If your accountant is suggesting that they need a TIN, there may be a good reason for that. I suspect he's thinking that there might be some question about this being an employee-employer relationship. If the IRS audits you, and if they find that you actually had an employee-employer relationship with this person, there is the possibility of some significant penalties.