Hi Anubhav,
The GMAC CATs are arguably the most accurate CATs available. However, the 'accuracy' of a CAT score is also based on how realistic YOU choose to make the CAT. If you skip sections, pause the Test, take it at home, take it at a different time of day as your Official GMAT, do anything else that is unrealistic, etc., then your score is likely to be less accurate.
During the last week before Test Day, a certain amount of your time should be spent on review - going over past notes/work, re-doing old questions, revisiting questions that you got wrong early on in your studies, etc. Working on some new material is also beneficial, as it helps you to keep your skills sharp. While there is some 'room' in terms of how much work you should do, you do NOT want to over do it. If you've been studying diligently, then a couple of days of extra hard study should not make a difference. As such, you'll be better served trying to rest up a bit more than putting in a lot of last-minute practice.
GMAT assassins aren't born, they're made,
Rich