Hi anurag16,
If you haven't taken a FULL CAT yet (in a realistic way), then you should take one soon. There are aspects to the GMAT that you can only learn by taking realistic CATs at regular intervals (so waiting until the 'end' of your studies to take practice CATs would be a mistake).
A CAT is really a 'measuring device' - when used correctly, it will give you a realistic score and help define your strengths and weaknesses, but it will NOT help you to fix any of those weaknesses. To raise your scores, you have to put in the necessary practice and repetitions. The CAT will show you whether your studies are helping you to improve or not. As such, you really shouldn't take more than 1 FULL CAT per week.
Many Business Schools expect applicants to score at a certain minimal level on the Essay (generally a 4.0 or higher) - so the idea that you would 'skip' the Essay during your practice is a questionable choice.
GMAT assassins aren't born, they're made,
Rich