In general I've found that since AHD 2.1, it's not necessary to always have the best Philosophy techs. In fact, I've found that you can gimp yourself pretty bad (in certain scenarios) if you're always grabbing the next +% research tech. In the year or two that it takes to get the next Philosophy tech, you could have gotten 2-4 industry (if you want to temporarily monopolize certain goods), commerce (if you want to fix your economy), naval (if you want to out tech the AI on the sea *this is a really good reason*), or army techs.
Mathmatically, if you want to have a broad range of techs, it makes sense to stay 1-2 levels behind in the Philosophy tree. Being a huge country with a ton of divergent priorities means that it makes less and less sense to sacrifice valuable army/navy/economic tech that you could have _right_now_ for a Philosophy tech that doesn't even do anything in and of itself. Although, it's kinda silly to have 5 levels of Political Thought tech. If you're going to go "high" as opposed to "broad" in the tech tree, you really do need to have more than 1 level of Philosophy.