I really depends upon the makeup of your team. If the team team is an 18u team that's been playing together since 12u, they probably won't require as much practice.
If it's a younger or new team, I think it is necessary to practice often before and during
the tournament season.
Of course, it's a good idea to have the Monday after a long weekend off ( parents need to do laundry, mow their lawns,etc).
Our 10u team will practice once a week and we play in a weeknight summer league with other travel teams. We will use practice to work out kinks from the previous weekend and to identify things we want to improve upon. The mid-week league is also an opportunity to give the 2's and 3's on the depth chart some quality work in a competitive setting.