The key for cracking BITSAT is spontaneity and time management. If you aim for CSE then you should definitely attempt all of the questions plus some of the bonus questions. It will surely fetch you enough marks, if you do solve them correct. In our batch, I personally do not know anyone who didn’t attempt those questions (there might be 2–3 though :p ).
Find out your best subject out of three/four, and make it almost perfect. You will have to practice a lot of questions and it is adviced to them in a time limit. Practice mock tests and see for yourself of you are able to complete it in time.
BITSAT will have 150+12 questions to be solved in just 3 hrs. So you will have to speed up a bit to attempt all the 150 and go for bonus ones.