Does the Planet Quality ever max out? As in, if you had a planet that was just land could you end up having every single square open for use with enough work/random events? I assume ocean squares never become usable...
All tiles can become useful. I once had a real monster planet, due to a random event. The event, that doubled the base PQ of a planet, happened to one of the planets of the Altarians. The base PQ was 29 (already the best in the game) turning it into a PQ 58 (!) planet. Add 3 more tiles, due to terraforming, and then another 3, because the Altarians build the Biosphere Modulator there. After I "acquired" the planet, I build a Weather Control Center (bought from the Arceans) there, adding yet another 3 tiles. Now, keep in mind, that I added 10% bonus to the Planet Quality ability to the techs Extreme Colonization, Biological Studies, Weather Control and Divergent Evolution and another 10% to the Orbital Terraformer. I had all of those, except Divergent Evolution. The terraforming tiles are not used in the calculation, as far as I can tell, but it wouldn't have been necessary, anyhow. Even a 10% bonus to PQ would have been enough, because the maximum number of available tiles is 72.