Yes,it is no compulsion, you can register yourself for counselling at any given round you wish. But with each passing round, the chances of you getting your choice of field in your desired institute will also decrease, as the people who apply for counselling in the earlier rounds and with good ranks are favoured more than the ones who register in the later rounds. You should definitely fill it in the 2nd round as waiting further will only harm you.

Hope this helps and all the best!!