Up until the last three months I did it the same way as you but after reading Active Directory Insights (Part 1): Configuring DNS on domain controllers :: Windows Server 2012 :: Articles & Tutorials :: WindowsNetworking.com and Best Practices for DNS Configuration in an Active Directory Domain | Dell Ireland
They now suggest-
DC1:
Pref : DC2
Alt : DC1
DC2:
Pref : DC1
Alt : DC2
If you have multiple sites (DC1 and DC2 together and DC3 remote) then they say-
DC1: pref- DC2, alt1- DC3, alt2- DC1
DC2: pref- DC1, alt1- DC3, alt2- DC2
DC3: pref- DC1, alt1- DC2, alt2- DC3