You can train stamina whenever you want. However, I would recommended that you train it either during the All-Star game week when usually most teams only have 2 games scheduled, or the last two weeks of the season, when, again, only 2 games are scheduled. For example, this week would be a great week to train stamina. To answer you second question, I think stamina is very important and prefer that most of my players have it at mediocre, at least. But this is debatable, and I'm sure other managers have different opinions.