The same room key privilege is available to Universal on-site guests--so I speak from experience with that.
We always charge to the room. I hate check out day when we return to the parks and can no longer charge to the room! We carry about $10-20 in cash in case we want a drink and a small stand either doesn't take the card (rare) or the system is down at that shop.
To avoid the "suprise" at check out, we keep track of all receipts on a daily basis. I keep an envelope in the room and each day survey the damage. The other thing we do to control spending is have a souvenier shopping day. Instead of buying everything we want when we see it, I write it down including price and location of shop. Then, when it is time to shop for souveniers, I look and my list, consider my budget and make purchases accordingly. It works for us.