I would always ask for the letter or get permission to contact the paediatric team as there may be information in the assessment report which helps to support the child, I also encourage the parents to ask for me to be sent a copy of any medication letters or diagnosis so we are kept informed of any changes.
If the parents want appropriate support (additional to or different from) for the child I would insist on a report, especially for ASD or ADHD where you may need to tweak behaviour policies etc. to support the child.
The flip side is that we have a number of parents who have ‘self-diagnosed’ the child because of a diagnosed sibling or a friend said so, which actually equates to a lack of boundaries at home.