Taking  a drop or not totally depends on your aim.

If you are determined to get into top NITs then you can take a drop otherwise, you should avoid it.

You should always have many options with you for that it is very important to give as many as entrance exam.

Taking a drop should be your last option.

Advantages

- You can prepare well for the exam as you already are aware of the exam pattern and level of the exam. You can solve many questions for your practice. It will give edge to your preparation.

- You can concentrate on your exam there will be no tension for your boards exam.

Disadvantages

- There will be loss of one year.

- Some companies do consider this gap at the time of placements.

- At times, you will get frustrated.

Good Luck!!