It depends on how you want to approach the ATP. Obviously you'll need it someday as you continue your career.

So you have two choices as I see it.

1.) Start studying now and use all the prep books as study guides. Instead of (or in addition to) memorizing the FAA test answers, make sure you understand the content that your being tested on. This approach will help you in an interview as well as taking a written test.

2.) Wait until you have an airline interview and then rush through the memorization of answers to abstract concepts that you may not have experience with & hope for the best in an interview.

Both methods could produce a successful interview, although I hate the stress of not doing everything I could to prepare. Method 2 is obviously more risky, and if it's a job you really want why risk it.

My advise is take the Boy Scout route and "be prepared".