I was under the impression that a zero offset rim was one having equal width either side of the parting face at the brake. Therefore negative offset has more rim inside this line and positive offset has more rim outside this line. As track width is measured from centre of wheel to centre of wheel, it's more important to stay close to the offset of the factory fitted wheel. ie if your rim is 1 inch wider, you want 1/2 an inch inside and 1/2 an inch outside the original rim. (if possible).
If you're not driving a manual you're really just steering.