At the airport immigration there are two lines: one for citizens and one for non-citizens. From what I "have heard" a Green Card holder should take the citizen line. Is this true for all international airports in the USA? If not, at which airports can the Green Card holder use the Citizen line?
You get in the line that says "permanent residents" on it. If there’s no such signage, you get in line with the non-citizens.
Depending on the airport I have seen permanent residents lumped in with citizens, sometimes with visitors. I imagine it has to do with the proportion of citizens to visitors at that particular airport/terminal and subsequent attempts at load balancing by the immigration authorities. The actual immigration stations are exactly the same regardless of whether the officer processes mainly citizens or aliens.
In any case, check the signage at the airport. When in doubt, ask the very nice woman in uniform who is usually hollering at the top of her voice at people to stay in line and to shut off their cell phones.
(As a side note, ever since getting Global Entry, I have been in bliss. Highly recommended).
You get in the line that says "permanent residents" on it.
This is certainly true. But if there is no such line, and there is no nice woman in uniform (nor anyone else from whom to get more information), and you are faced with the two options "US citizens" or "non citizens," then you should take the noncitizens' line, because a permanent resident is indeed a noncitizen.
An exception would be if you are traveling with a US citizen, in which case you can go together to the US citizens' line.