I've always admired the grace, efficiency and symmetry of swimmers that breathe to both sides at any pace and distance. Laure Manadou, Rebecca Adlington, Federica Pellegrini and many other elite female swimmers breathe bilaterally while competing, as do many excellent masters swimmers. But, many other women and almost all of the elite male swimmers in the world breathe to one side, or unilaterally, in their races.
Welsh distance swimmer Dave Davies is one of the few male swimmers I have seen consistently breathing bilaterally. World 1500m champion Sun Yang quickly breathes to both sides before and after turning, but mostly breathes to one side. Many male swimmers can sneak a breath to the opposite side to keep an eye on an opponent, but most opt for the additional air available when breathing every other stroke.
Despite the prevalence of unilateral breathing, some coaches recommend bilateral breathing to develop symmetrical body roll to each side and to avoid the lopsided stroke that often comes with same-side breathing. Michael Phelps breathes to one side, but in this training video, his coach, Bob Bowman, recommends learning bilateral breathing: