How to approximate figures while calculating income tax?

For calculating self-assessment tax I have to do some tax calculations myself. I have seen people making approximation in such calculations in different ways. Some people approximate to round figures at the end of each major step and some people do the approximation at the end of all calculations. Which method is correct? While calculating HRA exemption I even get decimal parts for some months. Should I round off to nearest rupee and then add HRA exemptions for all months or should I add all such figures with decimal places and finally round off to nearest rupee? Which method is correct? It will be helpful if you refer me to some sample examples which explain these issues. Thanks.