I think the Park TM 1 at ~$60 is a very good investment if you plan to build more than one set of wheels. And I agree with most others that you really don't need it but it sure is nice to set the drive side tension on the rear wheel as high as is practical so that you can set the non drive side high enough to avoid spokes breaking from insufficient tension and still center (dish) the rim.
As far as economics are concerned I'd rather spend reasonable money on a tension meter than on a dishing tool. You can center the rim accurately by reversing the wheel on a truing stand.