Breath control is necessary, sometimes.

In classic technique the shooter hyperventilaltes slightly, then exhales 2/3 of capacity, holds breath and executes the shot before anxiety, dizzyness, and blackout occur. Should any of those symptoms appear, the cycle is repeated.

Timing, as in learning to control breathing, align sights, squeeze trigger, and follow through on 1/3 of a breath is the goal.

In modern marksmanship where mechanical support is allowed in matches, and mostly required in hunting or operational situations, breathing and breath control may not be performed quite the same way.

If you are breathing heavily from exertion then holding your breath for any length of time is a waste of effort. If you can maintain your sights on target indefinately (due to correct support of the rifle), then breath control might simply be to stop breathing long enough to make the shot, and resume natural breathing after follow through (without the preparation of hyperventilating).

Sometimes it is critical the shooter be able to communicate while observing and just prior to the shot, again violating the classic breath control regimen. Practice the classic breathing technique until it becomes second nature for you to pause as you shoot. This will handle all but the 'out of breath' scenario. In that event, you will likely find your best accuracy comes from utilizing the free recoil technique, specifically so that your elevated respiration and heart beat do not infuluence the rifle.