I purchased a placeholder with a gift card; however, I didn't try to do it via stateroom credit. I don't think that they offered the option of charging the placeholder to your stateroom.
There is no real reason to put more OBC on your account than you plan on spending. As long as you are keeping the balance below the threshold to get charged to your credit card, you can 'pay as you go' during the cruise. For our last cruise, I put an initial amount of OBC down off of a gift card, which we burned through fairly quickly with Palo, spa for my wife, bingo, shopping, etc. If we spent a lot, I'd add more, and then on the last night I got a balance and paid exactly the amount due with another gift card. Call me conservative, but I'd rather pay what I owe than risk any issues with getting a refund on the correct gift card.