Sql clustering sql server account
All nodes in a cluster should be using the same service account for SQL Server. This is true for both an FCI instance and an Availability Group setup. 

For security sake, consider using Group Managed Service Accounts.https://blogs.technet.microsoft.com/askpfeplat/2012/12/16/windows-server-2012-group-managed-service-accounts/

This will allow you to have a service account that will have an strong, automatically changing password for your SQL Servers. Auditors love this. Your risk is minimum since Active Directory is handling the password change and downtime is not required to do this.