While I'm sceptical of this kind of hidden effects, I'm also superstitious enough to actually do this. I'm playing one of my best seasons (in the league) and I've had a few players always with only "very good" morale, first just to save blue packs, but now consciously leaving them just in case. And my win bonus at half. My only loss and 1 draw (of 4) came when I maxed the win bonus.