Venus Fly Traps respond well to being repotted once a year. The media can get old and compacted making it hard for the roots of the plant to grow. Repotting will assure that the media stays fresh and aerated.
The best time to repot your Venus Fly Trap is in the early spring time just as it is coming out of dormancy. The only time to actively avoid repotting a plant is when it is flowering. However, Venus Fly Traps don't seem to mind being uprooted at any time during the year and usually resume growth within a few days of being repotted.
If you just got a plant from the store, I would recommend repotting it as soon as possible. Usually plants in stores aren't cared for properly and are watered with impure water. These impurities will stay in the media. They might be able to be flushed out with a heavy watering, but it is just easier to mix new media and repot.