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.
For this you can make any field non nullable and offer a default value for the field.
Counter example: SAP HANA doesn't support NULLs, and as a result if I see 0 in any column in the database I have to wonder was it 0 that user submitted or 0 that system added as a placeholder. I would need to go back to documentation and see if the field is mandatory etc. Better have it explicitly stored in the data itself.
That interpretation makes sense to me when I read the docs, but is way too easy to miss when I'm actually doing comparisons.