Compared to university system (where you get percentage), it is easier to score in IITs (I don't know about BITS), because the teacher who teaches you makes the question paper, so the possibility of something being out of the "syllabus" is negligible because the syllabus gets decided by what the teacher has taught.
It is better to give your class rank in your application so that the universities in US have an idea of how well you did in your own college. There is no such magic common cutoff for all colleges. Obviously, the college name matters during admission, as in, they are more likely to offer you admission if they have had previous good experience with students of your college. IITs (and BITS, to some extent) are quite popular and almost all engineering professors in big universities know about them.