It can be confusing but for the "Run As User" account when you first start to setup Tableau Server we actually recommend not using an administrator account:
Under the best practice section we have the one bullet point:
Do not use an account with any kind of domain administrative permissions. Specifically, when you create an account in Active Directory, create an account in the domain User Group. Do not add the account that you create to any Active Directory security groups that needlessly elevate the permissions for the account.
You can for the most part select an account you feel would be appropriate