Rotating is always a good practice. The ends of the bulbs generally don't put out as much light as the centers, so that will help keep them as close to even growth as possible.

Light intensity drops off with distance rather drastically (Intensity is inversely proportional to the square of the distance), so the cool bulb's light isn't really getting to the side under the warm bulb side as much and vice versa. I don't think you need to rotate them daily, but at least weekly would be a benefit (or certainly won't hurt anything). I normally rotate mine every 3 or 4 days.