Ben
I would be rather surprised if you had to do that, and somewhat nervous of making any cuts into a steering component.
The track rod ends are the only other place where you can affect the steering, Check there is the same thread showing on each track rod. By counting the turns you could center the wheels without changing the toe-in.
Of cource this assumes that the rack itself is centered.
If you have the alloy rack this should be impossible to be offcenter - if it is the old steel rack it is possible for the rack to be offcenter.
Nick