In the manual it states that a civilization's planet quality bonus affects the number of usuable tiles on colonized planets, but this only seems to affect the planet that the civilization starts on. To check this, I boosted my civ's PQ rating to 900%. The starting planet had all usuable tiles, but no planets colonized later were affected.
Also, changing the PQ rating back to 0% in the same game did not reduce the number of usuable tiles on the starting planet, so it looks like the game is only calculating usuable tiles when a planet is first created.
Is this how the PQ rating is supposed to work?