In standard OpenERP, you can choose a fiscal position for every partner, and your choice will be used when you select a Partner in an Invoice or Sale Order. In many situations it’s convenient to set a default position for all partners belonging to a specific country.
Go to Sales > Configuration > Address Book > Localisation > Countries.
Choose a country, and enter the default Fiscal Position for that country.
Done that, when you create a Sale or Purchase Invoice and select a Partner, the Fiscal Position you defined for the country of the Partner will be selected. Note that the standard OpenERP behaviour is still defined: if you set a specific Fiscal Position for that Partner, that one (and not the one from the Country) will be used.
To get the same functionality in Sale Orders, install the module account_fiscal_position_country_sale as well. The modules are kept separate to minimize dependencies.
Of course, feedback is welcome! You can use the bug tracker in launchpad for that: