I assume you started in the center of your quilt? I agree that some wobbliness won't matter. However, if you really want to start compensating a bit for the "out of true", what I would do I use white chalk (or white soap) to measure out from the center to a little beyond your last quilting line. You can use a string to mark a circle or, if it's easier you can just mark a square. Mark again a few inches beyond that, and again all the way to the edge of the quilt. These would not be quilting lines for you to follow. Rather, they will be marked guidelines so you can gradually compensate for the lines you have already done. For example, if you find your last quilting line is close to a mark on one side but far away from another side, you can quilt a little closer to your last line or a little farther away from your last line to start making up the difference.
Hope this is clearer than mud.