Many swimmers prefer to breathe to one side over the other side. I prefer to breathe to my right side and almost exclusively breathed toward the right during races. In the instances where I did breath toward the left, it was usually just a quick breath to check out the field and that was it.
I think it is important to stress that the way I breathed during my races is different than the way I breathed at practice. At practice, I performed bilateral breathing (breathing to both sides) because it kept my stroke even.
A lot of times, swimmers who only breathe to one side have what I call a lope-a-dope stroke, meaning that their stroke rate is uneven due to one arm cycling faster than the other. In some cases, shoulder injuries can result from the combination of overuse and uneven strokes. The more symmetrical a swimmer’s freestyle is, the less likely they are to injure their shoulders in the long run.
To practice bilateral breathing, pick an odd number breathing pattern and stick with it for a set. When I say odd number, I mean breathing every 3 or 5 strokes instead of every 2 or 4 strokes. You are welcome to mix things up, just keep the pattern odd at practice. Here are a few examples (without heart rates or intervals) of what I mean when I say you can mix the breathing pattern/ breath control up:
5 x 200s Pull, B.C. 3/5 x 100
6 x 75s Free, B.C. 3/5/7 x 25
1 x 300 Free B.C:
3/5/3/5 x 25 in the first 100
5/7/5/7 x 25 in the middle 100
7/9/7/9 x 25 in the last 100
4 x 50s Free Breathing right/left/front x 3
Notice I included front breathing in the last example. I did this because It is also good for triathletes to practice front breathing, which is called sighting. You want to do this because it helps you pick a point and swim in a straight line during a race.
There we have it. Use bilateral breathing to keep your shoulders healthy and your stroke even at practice and do whatever breathing pattern feels natural during the race.