Is it a good idea to take a drop of 1-year to pursue B.Tech in CS, IT, EC from IITs and NITs?

If you have confidence that you can get CS branch in any of the central government colleges then only I'll suggest you take a drop of 1-year, otherwise, not as you can also see what is the condition of the engineers in day to day life.

IITs and NITs are no doubt best institutions in India if you can crack JEE both mains and advanced. I think no matter if you lose a year because you are going have foremost importance and preferences.

Yes, it's a good idea. If you want to do engineering and if it's your dream then just go for it. Work hard. Good luck.