If Net Taxable income is less than Maximum amount not chargeable to Tax than no one is require to deduct tax.
Net Taxable Income = All incomes - Loss from House Property - Deduction Under chapter VI (ie Sec.80 deduction)
Note : You can not consider Deduction U/s 80G while calculatin TDS on salary.
The bsdic duty of the employer is to get all the details of sources of income of his employee and the investments made by him and if his salary is not exceeding the exemption limit then make sure his net taxable income id not exceeding the limit so as to be sure as to whther to deduct tax or not...!!!