But... I'm beginning to wonder... you don't suppose tax rate and population growth rate might be inversely related, do you?
I originally thought that, but they're not. I was lucky enough to have 3 planets formed on the same turn with the same amount of arcs. So I had one with 20% tax, another with 10% tax, the other with 0% tax. There was no difference in population growth. I then thought it might be linked to resource production, but again, it isn't. I might try again when they have much larger populations in case the effect is only small.
The only reason for having a lower tax rate is if you're running a deficit, that way you can reduce the amount you're paying out so you only need to build fewer trade ships to make a profit.