AWS employee here.

You are welcome to check with AWS Support based on your predicted traffic load for extra assurance, but in my experience the new ALB no longer requires priming, even for significant traffic spikes.

If any of you are going to Dockercon in Austin stop by the AWS booth and I'd be happy to show you a demo app that uses an ALB with about ten rules to route traffic to different microservices running as tasks under EC2 Container Service. The demo includes a load test to drive traffic to the ALB and the services behind the ALB, verifying the consistent performance as traffic scales up.