Are reward box contents set when you obtain them or when you open them?

Question

I've begun receiving reward boxes in my inventory, and am wondering if their contents are determined when I obtain them, or when I actually open them. Their description says "This box must be opened to reveal its contents. A box holding an assortment of items."

So, for example, would the contents of a box obtained at level 5 be of higher value if I waited to open it until I'm at level 10?