You mean, gift card balance or Amazon Payments-->Bank account, right? I think it's considered wages once it's available for transfer but, I could be mistaken. And gift cards are still considered payment for work performed. So, it's income once it's given to you, regardless of whether you withdraw it at all.
I could be wrong.
Anyway, Everything you never wanted to know about taxes and Mturk.
https://www.mturkcrowd.com/threads/taxes-for-turkers-reporting-self-employment-income.127/
EDIT I always transfer to 'Payments'. You can pay for stuff on Amazon directly from 'Payments' balance, or withdraw it.