+1
Use of a subdomain prefix is absolutely the right thing to do, trendy root-domain-only sites notwithstanding.
CNAMEs are inherently more flexible and more resilient in the face of various load challenges or DoS attacks:
"Root domains are aesthetically pleasing, but the nature of DNS prevents them from being a robust solution for web apps. Root domains don't allow CNAMEs, which requires hardcoding IP addresses, which in turn prevents flexibility on updates to IPs which may need to change over time to handle new load or divert denial-of-service attacks. We strongly recommend against using root domains. Use a subdomain that can be CNAME aliased... "
- Heroku [https://status.heroku.com/incident/156]
CNAMEs are inherently more flexible and more resilient in the face of various load challenges or DoS attacks:
"Root domains are aesthetically pleasing, but the nature of DNS prevents them from being a robust solution for web apps. Root domains don't allow CNAMEs, which requires hardcoding IP addresses, which in turn prevents flexibility on updates to IPs which may need to change over time to handle new load or divert denial-of-service attacks. We strongly recommend against using root domains. Use a subdomain that can be CNAME aliased... " - Heroku [https://status.heroku.com/incident/156]