Don't know what type of mill yours is, so it could have a specific tramming procedure. When I trammed my WM18, which is a very common vertical column mill design, I could not get the head nod right and there's no adjustment for that. The only solution was to shim the column as mentioned above or shim the head. I opted for the latter. The only drawback I can think of if you do it this way is you have to re-shim if you need to rotate the head, but I will not be doing that inless it is absolutely necessary.
How often to do it I think would depend on how you treat your mill, a deep doc with a flycutter might knock it out of tram.