I would go to the extent of saying that the test series is indispensable, given the fact that General Studies (GS) weightage has grown and securing good marks in GS is extremely important. I have enrolled myself for Sri Chaitanya Narayana test series in Hyderabad and took the question papers and model answers for “the-most-famous-test-series-of-Delhi” from one of my friends. I used to take the exams for the latter test series at my home and used to self evaluate them looking at the model answers. I would strongly recommend both the test series to all the aspirants and it helped me in both managing time as well as improving writing speed in the writing marathon of the mains.
I believe that studying would constitute just 50% of the preparation and writing tests would comprise the remaining 50%. Test series teaches many important lessons and allows you to identify and rectify your weaknesses so that you do not expose them in the final exam.
It is also important to try and attempt all the questions in the exam and it is quite possible to do so. In all my GS papers (I, II, III and IV), I have attempted full 1000 marks without leaving out even a single question. People say that it is impossible to answer 25 questions in 3 hours time, but I would say with correct practice it surely is possible (not to forget that I also drew maps in geography answers). I think the only reason why you should not attempt a question in the exam is only because you have absolutely no clue of what the answer is and you can’t even make an intelligent guess of what that is. I feel no other reason is acceptable.