Not necessarily training paint mate. Ive shot thousands of boxes probably, of all different kinds, brands, qualities etc, and you get one slightly mishapen ball in a bag in your box of round paint, and thats the ball that loses your tournie.As a team we always barrel match paint and sometimes during the day if we have a big change in temp during the day. (paint can change size during the day increasing muzzle velocitys!)
I personally use a tight bore on my Geo (so you can just about blow it out with your mouth) and I have found it improves its accuracy, efficiency and shot to shot consistancy. I also have no probs with breaks.
However this is as long as your using good quality paint. If your using cheaper training paint (which by default is normally more inconsistant) I would use a larger bore and put up with a slight lack of Accuracy and efficiency in order to combat the higher chances of barrel breaks.