Farms are easier to use on high quality planets because you get a positive morale effect of high planet quality, as far as I know this is the only aspect where planet quality comes into the equation.
The +10 approval bonus you get for a planet higher than PQ10 is not going to make the difference between whether you have 14B people or 20B people on that planet.
You lose 26% absolute and your efficiency drops from 60% to 34% on morale which means that to have 100% morale at 20B you'd need 195% (counting the +10 approval bonus), or 203% without counting it. I haven't factored in taxes, so it's not quite as bad as needing double the morale, but it's still bad-bad enough that +10 approval, even when it isn't depreciated by population, isn't going to make or break it.
The number of tiles you have left over for virtual reality centers (or their equivalent) is a primary factor, and how many you actually need is even more important.