If you qualify mains with good score then it's on you if you statisfied with NITs then there is no need for preparation for jee advance.If you interested in IITs then you should do.Qualifying score and score required for seat allocation is different,You need higher score than qualifying score to get seat in NITs.

You should prepare for 2nd pahse of jee mains and for advance examination.As only percentile score is declared by NTA .Which only tell about relative performance of candidate.You can ask if you have any other questions.

BEST OF LUCK!