Most swimmers have a natural breathing side — normally the side of the dominant (writing) hand, but not always. It’s fine to exclusively breathe to this side because it’s important that your breathing fits into your rhythm, the same way it does on the run and bike legs. Come race day, finding a rhythm is important, so nothing should feel forced to you.
Having said that, while training, it’s good to experiment with breathing to both sides. Doing so helps reinforce “balance” and a full rotation of the hips. (It’s also possible that, over time, you find that breathing bilaterally (to both sides) fits better with your natural rhythm.)
One of the best times to practice breathing bilaterally is during pull sets, where you are using a pull-buoy. By isolating the pull and floating the legs, you can focus on finding a breathing rhythm that uses both sides. Consider swimming a set of 50s, where you breath every 3 on the first length and 5 on the second.
If you haven’t done this before, it will take some getting used to. There’s a good chance that you will swallow a bit of water. We all do when learning this.