Overly assertive bosses can be seen as bullying and over bearing. But bosses who tend to hold back may be considered wimps. Good bossed find a balance between the two.
This does not mean you should try to be assertive all the time. Instead, be prepared to use both approaches in different situations. Your team members may need you to challenge them to accomplish a particularly tough goal. Or you may need to be more passive to let them step up.
Be flexible. Use your emotional intelligence to determine when being assertive will be motivating rather than stifling, and laying low will be appropriate rather than discouraging.