In our experience you can't "preload" your room at check-in. You can only pay down your balance with Gift Cards. You don't have to do it daily but you do need to do it prior to your checkout day or they may charge your credit card. On our last trip we did it twice, about halfway through and the night before checkout.
Also - depending on which cast member you get it can take awhile for them to apply these. Each time we were using multiple gift cards (I was paying down our balance of approx. $500 with 5-$100 gift cards on two separate occasions). The one night it took about 5 minutes and they were done. The other night it felt like almost half an hour. It could have been a slow computer or just a CM who hadn't done it before. I was fine with it as the kids were getting ready for bed back at the room anyways but I think the people waiting behind me to check in at 9pm were a little frustrated.