Hi,
I agree with Pallu that a minimum level of 5-8 for secondary skills must be good for any player (unless you want to create a Rodman or a Wallace).
Looking at the official instructions I've seen that the skill "passing" is broader than I thought. It is obviously related to passing the ball, but it is also related to receiving assists. Thefore, I think it must be a pretty important skill for a center (especially if you have good passers in the team and play inside tactics). Furthermore, it will help my team reducing the number of turnovers (the 16 on average that I currently have are ridiculous...), so I will train a bit my two big men next season on passing the ball.
Jordi