As you have asked that, after availing deduction u/s 80C, if you fall below the thresh hold limit then should you file income tax return or not, then the answer is yes.
For e.g. If your annual income from salary is Rs. 4,50,000 and after availing deduction u/s 80C of Rs. 1,50,000 your net taxable income comes to Rs.3,00,000 (4,50,000 - 1,50,000), in such case your will get rebate of Rs. 5,000 u/s 87A and your income will not be taxable.
Although your income is not taxable but you should file your return.
Anyway one should file NIL return because at many times in case we require any loan, then they (bank/financial institutions) do ask for return of 3 years, so it will serve as documentary evidence that you had been working and had been filing your return for past years.
Hence in the referred situation in the question, Income Tax return is to be filed.
Income Tax Act, 1961 states that an individual or any other person as defined in the said Act shall file Income Tax return if his/her income is above the thresh hold limit.
Filing Income Tax return would be beneficial for you in future, because if you need bank loan for house-building or similar other purposes, you can submit the copy of filed return (ITR-V) as an evidence of your income.
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In such cases it is advisable to fill it regularly so that whenever it exceeds the limit and a refund is due, the return will be processed quickly.
So filing of return regularly is a good practice irrespective of fluctuation in income around the threshold limit.
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Because as per the provisions of income tax act,1961, every person whose gross taxable income is exceed the basic limit is compulsory to file the return of income.
however it is given to relief to taxpayers to get deductions under chapter VIA (80 C deductions are covered under this chapter).
so every person must file the return of income despite of there income is not taxable after getting deduction of chapter VIA otherwise the department will issue a notice that why did you not file the return of income of the particular year.