Another consideration: pq 14s I usually grab, occasionally grab early for production bonus during the planet rush to produce colony ships, and then sell in mid-game when I seek to keep my morale up for pop bonus. Especially useful are systems with two or even three pq 14's, which with only a few turns can be cranked; with hard social spending they can become 17s (or better). Those star systems, with a few billion pop each, make VERY handy trades to the I-league, cos they pay huge sums of money for them and are almost always willing to trade every tech/trade good they have for them. Other minors work almost as well. If you have the pq+ trait, they'll sell even better as they'll be 18+. This strategy has netted some very nice cash to win the wonder/tg races, as well as funding wars.
For example, at one point I had the Alexians paying me something like 500+ tribute per turn for maybe 100 turns for a stack of tech I gave them. About 3 turns after I made the trade, they were attacked by Yor (or maybe Drengin, don't remember). Of course, with their economy devastated by the mortgage note, they had no chance. To save them (to save me) I simply traded the planet to them (for maybe 40 bc for 200 turns which was all they had), which was well defended in my territory, so they could continue paying me off. 50,000 or so bc is certainly not a bad trade for a few pq 14s...