IIRC, the city with the FP is capped at a maximum of 30% corruption, irregardless of gov't, OCN, rank, distance or any other factors. Under Communism, the city with SPHQ enjoys the same cap. The cap is reduced to 20% with a courthouse, and 10% with courthouse + police station. This may or may not affect where you put them.
Usually I try to build the FP in the 2nd ring away from my capital, towards where I expect more growth via settling or conquest. The goal is to get it built quickly. I rarely bother with Communism and SPHQ, however when I do I generally place it in or near a former AI capital. Preferably this is an AI that I have utterly vanquished and I therefore do not need to starve down the population. Such cities are typically large and fully developed with OCP spacing that helps to maximize shields and commerce.