No, you do not have to have a job in the field to take it. I'm sure it helps.
I purchased the AAPC practice tests and did them repeatedly. I took my exam yesterday
and the time pressure got to me, to be honest. I spent way too long analyzing my answers
in the first half, only to be rushed in the second half. I also made dumb mistakes on easier stuff, like ICD 9 codes, because I was rushed.
I likely didn't pass but that is ok, I know what to expect now and what areas to work on.
Also make sure your book is marked up and tabbed, tab the anatomy graphs, and make notes everywhere. You'll be surprised how you don't think as clearly when the clock is ticking, so the easier you can find something the better.
If you are running out of time, skip to the back and answer those questions first. They are terminology ones, etc and take less time to answer.