Andrew:
In Invoices blank zero's is not really a spec file solution. In the company profile there is an option to "Text for zero amount" where text can be blank.
This makes all $0 amounts blank for all spec files, there is no option for specific fields to be blank or only specific spec files to have blank amounts.
We have this need frequently based upon some condition, often multijurisdictional taxes, where blank all $0 would not work (we only want to blank the Tax amount when no jurisdiction exists.