[sjwright]

You should always think about NULL state for every field when designing your schema. If you don't have a clear sense of what the NULL state shall mean for a given field, the field should not allow null values.

For example: a shopping cart might contain a tee-shirt in blue colour and expedited shipping in NULL colour. That blue tee-shirt might be the same colour as some blue socks... but that NULL-coloured shipping isn't the same colour as a NULL-coloured charitable donation.