Hi Midhila,
Most Test Takers will have the most energy early-on in the Exam - so as a general rule, it's a good idea to take the Quant and Verbal sections early-on (as opposed to taking them after the Essay and IR sections, when you might become a little tired and not perform at your best). That having been said, if you can keep a high energy level for the entire 3.5 hours, then it's possible that taking the Essay and IR sections might help you to settle into the proper Test 'mindset' for the later sections. Each practice CAT that you take provides you with a great opportunity to measure your GMAT skills under proper testing conditions - so you have to put in the extra effort to make each of those experiences match-up with what you will face on Test Day (take the FULL CAT - with the Essay and IR sections, take it away from your home, at the same time of day as when you'll take the Official GMAT, etc.). If you plan to take the GMAT sections in a different order, then you should try that order out during practice - and train for it.
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied? How many hours do you typically study each week?
2) What study materials have you used so far?
3) On what dates did you take EACH of your CATs/mocks and how did you score on EACH (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
4) What is your overall goal score?
5) When are you planning to apply to Business School?
6) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich