When potholes put flat spots in your rim, they are often not conducive to truing in the conventional way. The rim will actually become wider at the point of impact and may rub both brake pads simultaneously, so shifting the rim left or right with spoke tension will not help much. And the wheel will be out-of-round in the radial direction, ie. a small section will be flat instead of round. It may be possible to narrow the wide spot with a special tool or even a hammer (do this on steel rims only). Obvously it becomes time for a new rim when the flat spot is too large, because the ride gets bumpy and the tire may not seat properly. I replaced one of my rims last year and one 2 years ago because I'd put up with flat spots long enough and I'd had new rims sitting around for awhile. Then I had to build a new rear wheel for my daughter's bike because it got stolen.