Its important, for end of games, your player wont perform as well if he has 2 stamina, compared to someone who has 8.
Also some people can start to foul more late in games if they have low Stamina due to them being tired.
I would say at least 5 is good, I think the sweet spot is around 7, mainly because if it gets to around 9 players tend to stay in for 40+ minutes a game, which makes it hard to manage minutes.