Hi,

No, it won't affect your job or your placement if you take a drop year for preparation of IIT. However, I would recommend you to join any local college for that certain period for that drop year as it will be like a valid reason when if someone complains about you having a drop year. You can explain them that you did 1 year college and then decided to give IIT exam. However, even if you don't join any local college it still won't have any impact or affect your job in any way.

I hope this helps,

All the best