I run premium because it's recommended by MB.
No fuel-related problems yet, but that may not have anything to do with octane quality.
If ever I do have a fuel-related issue, at least it's a parameter I can rule out.
Many on this forum used regular (81-85) and have had good results.
Higher octane fuels have a higher "flash point" i.e., needs higher temps in order to ignite. So there is less of an issue with problems such as "pre-ignition" or detonation.
But if you start hearing pinging under load conditions, then you do need to bump up the octane choice.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car