When designing the initial model for a database, I'd rather err on the side of tight constraints (ie: default to not-null if the business can't think of a good reason it would ever be NULL). If the business later says: "Hey, we get an error when we try to enter a PO for a tax exempt customer!", then it's no big deal for the DBA to deploy a single line of code to alter the constraint. Frankly, that's just spilled milk.
However, if you default to null, and the business says: "Ooops! We entered several dozen PO last month without adding sales tax.", then that's potentially a big mess to straighten out for everybody.
"Do not seek to follow in the footsteps of the wise. Instead, seek what they sought." - Matsuo Basho