They do seem to prioritize paying customer, particularly during outages.

https://status.heroku.com/incidents/151

"We prioritize getting top-paying customers back online over our larger base of free users, which is why customers (particularly those with dedicated databases) were back online much more quickly than free apps. While we think this prioritization makes sense, we do strive to provide a high level of service to everyone. Even though the outage was much shorter (less than 16 hours in most cases) for our top customers than for our free users (as much as 3 days in some cases), we measure our downtime as the time it took to get 100% of apps back online."