Morale 1: 40% compounded Maintenance
Morale 2: 35% compounded Maintenance
Morale 3: 30% compounded Maintenance
Morale 4: 25% compounded Maintenance
Morale 5: 20% compounded Maintenance
Morale 6: 10% compounded Maintenance
Morale 7: 0% compounded Maintenance
Morale 8: -10% compounded Maintenance
Morale 9: -15% compounded Maintenance
Morale 10: -20% compounded Maintenance
The game is balanced such that average morale 7 is fully survivable. This allows you to abuse Quality to a certain degree.
However, Quality overruns cost additional Maintenance and can drop Morale as low as 6. So there is a need there for careful management and Spice Festivals.
I have some substantially over-quality worlds in my Empire (10 too many buildings) that function just fine if they're carefully cared for. Be aware, however, that the rules fully cover this scenario and you pay a scaling increase in Maintenance penalty for every 1% over Quality you go, so a 10 point Quality overrun on a 32 Quality planet generates about 30.5% Maintenance penalty.