Whether you have joint custody agreement or sole custody agreement, the information I'm going to give you is still going to help especially when drafting your parenting time agreement.
My answer is if you share joint custody this is not a good idea at all to go against the other parent's wishes. You need to have the consent of the other parent or a court order to do that. In some cases I have had parents make major decisions for their children that the other parent was not in agreement with and it actually caused them to lost custody of their children.
When it comes to decision making or custody it usually pertains to major decisions for the life of the children. For example mental health treatment, medical treatment, education, in some cases religion. In Arizona we have personal care decisions. Which amounts to for example can the child get their hair colored blue or a mohawk or get all kinds of piercings.
If you share joint custody with another parent then you have to discuss those categories of major decisions with that parent before making one of those major decisions. Certainly enrolling your child into some sort of mental health counseling would be a major...