Depends on which theories exactly you mean. Some of this conspiracy stuff is utter nonsense but some others are at least partially true. It's always dangerous to generalize in such a way. And you always have to keep in mind that there are and were indeed real conspiracies. At least it has been proven numerous times that officials lie to us big time. Best example for me is the Nayirah testimony. So.... In my opinion you should always listen to those theories. Luckily most people are quite capable of distinguishing the bullshit from the true problems like corruption and stuff.... Oh and I recommend you the book Propaganda by Edward Bernays. He is known to be the "father of modern PR".