That's what we do and it works fine EXCEPT:
- I can't 1-click other purchases because it will automatically take the money first from the GC balance
- I have to go through checkout for physical item purchases so I can click "change" on payment method and uncheck "use GC balance"
- I wouldn't count on seeing Kindle purchases reflected on the gift card immediately. I usually see a 15-30 minute delay between when I click on any known paid books and when it shows up on the GC balance page.