higher PQ == higher population == higher tax revenue == more $$ towards production
So if you populate a low quality planet, pump it up with banking imps and morale improvements (to keep the masses happy). You'll get shafted on Influence, though.
Boogie, I disagree. Tax revenue and production have nothing in common. I often use tax = 0% (trade for 100% of income).
The equation is something like this:
Tax revenue depends a lot of population, and a little of PQ.
Production depends a lot of PQ and marginally of population.
I think Ellestar has put the equations up somewhere (must be on apolyton).
PQ lower than 15 are worth almost nothing in terms of revenue (it's just the population that will generate money, and you can't have a lot of pop there). In terms of production, they are poor, but can be improved up to 4 times PQ production before starbase/improvement bonuses start becoming less effective.