This is obviously part of the shooting routine, but to address a non-routine holding of breath--don't do it. I think this happens even with experienced players when attempting a "low-confidence" shot, it leads to a tightening of the body, which dominoes into bad execution. For example, when trying to pull off a power-draw, when it has not been practiced to an acceptable level of success.
IIRC, Wayne Rainey, a former Grand Prix Motorcycle Racing champion, considered holding one's breath as unnatural for the body, so not breathing (while riding/racing) was bad form. Possibly from his book, "The Soft Science of Roadracing Motorcycles", my riding buddy would quote to each other, "Remember to breathe". We did this whenever we were attempting something in which we were unsure of the result--motorcycle related or not.