> That said, it's a trade off between efficiency and load spike tolerance. I trust that the trade off is made with informed decision.

I don't think that relatively low utilization rates is the scenario that requires "informed decision". The only tradeoff in low utilization rate scenarios is cost, which might be outright cheaper and irrelevant once you do the math on the tradeoffs of using reserved instances vs the cost of scaling up with on-demand instances.

You need to make a damn good case to chronically underprovision your system and expect it to autoscale your way into nickle-and-dime savings.