Return filing not necessary for income below Rs. lacs
If your income is below exemption limit specified by IT dept. without deduction of 80C - 90U, 10B,10A and 10BA then you need not file income tax return. For assessment year 2012-13 the minimum exemption limit now is Rs. 2 lacs(Read here) and last year it was Rs. 1.8 lacs.
No need to file for salary income below Rs. 5 Lacs
Otherwise if yours is only salary income( plus bank interest but without any other income like house income, trading, business etc) and your income is exactly reflected in your Form-16 given by your employer then you do NOT need to file income tax return for AY 2012-2013. This relaxation was given last year also. So filing income tax return is mandatory for those having gross income of Rs. 5 lacs and above.
So it is not mandatory to file if you meet all of the following rules:
Your gross income is less than Rs. 5 lacs
You've had only single employer in the last year
You have only salary or bank interest income
Bank interest income is less than equal to Rs. 10,000
You've no tax outstanding and all is fully deducted by your employer
