I have an inverter tig welder too. Weld with +earth for steel in DC, and for aluminum in AC.
If you use - earth on most welding, you'll smoke the tungsten.
Only time I know of that -earth is used is for super thin/very low power on steel. Beer can thin stuff; when you need the heat to go to the tungsten instead of the metal you're welding.
Have fun, post pics of the project!