See if you have got a seat allocated in some round say x,then you have two options available either lock that seat and that college or leave the round so that you can appear in the successive round.if you go to the next of counseling then the previous seat will no longer be retained and moreover you cannot go back to the same seat if you are already in the next round.hence be very careful in your judgement and choice locking process.
You will be eligible for counseling up till the last round.but what happens is all good colleges are allotted upto round 2.there is no point going up to the last round.moreover you'll be leaving all the seats if you go upto the last round.