Please refer to the page 10 of the following document:
The document clearly says that if you are citizen of 5 designated countries then you should go for the registration every time you come in.
Other clause says that if you get selected when you re-enter, then you need to go for the special registration procedure.
First of all I am not from thOe 5 countries, secondly I was not selected at the port of entry. The immigration office was pretty polite with me when he came to knew that I have done my Master's here and now working for a consulting company. He saw my license and asked me few questions which he entered in the computer. Even though my passport was not stamped, my boarding card was stamped saying Arrival in USA. I believe I dont have to go for registration. But from the vaulable forum, everybody is providing me with suggestion that I should do that as burden is on me. I will definitely call the Local INS office tomorrow and find out whether I need to go for the registration. I will update what is going on. I am kind of reluctant in doing this as it will cost me almost the entire day of my work. But thank you all for your suggestions.