Its no longer optional but mandatory to have a TIN.

Effective 1st Dec 2017, all employees who earn 235,000 and above, must include their TIN during PAYE filing. Its now an obligation of an employer to make sure that all people he/she employs have TIN.

Note that, to claim salaries as your business expenses, you should be filing PAYE monthly for your employees. Otherwise, you risk having the money paid as salaries being dis-allowed during income tax computation.