I'll generally use my leader for the sistine chapel. If i miss it i'll either lay down bachs or go for the FP. If i'm lucky enough to get a second leader, i almost never get it before the late industrial age. By then my cities are so productive that i can build most any wonder i want quickly enough (i cash rush my factories/coal plants) that i feel silly using a leader. I don't see much usefulness in armies.
My FP is usually built a bit outside the fringe of my first ring of productive cities. There, the cities are not overly corrupt and can usually complete it in 20-30 turns. This gives the benefit of helping out my other fringe cities, bringing them from 30-50% corrupt back to useful enough to create the strongest military unit of the era in 2-6 turns. Whether its in conquered land or not just depends on what the random map dealt me.
Although i generally try to build the FP as early as possible, the option usually doesn't present itself and i end up building it in the late middle, or early industrial ages. I think 6 content faces (from the cathedrals-sistines) can outweigh the increased production from the early FP, allowing me to be the aggressor as a republic/democracy and invest no money in my luxury taxes.