I agree generally with what people have said here, but I do think that taking full PTs is valuable as more than just an indicator. I'm doing a lot of full PTs (I'm a Dec. taker) because I want them to feel like the back of my hand, in terms of stamina and timing.
I am definitely always going back and studying what I've done wrong, but honestly, if you are scoring very well, there aren't enough missed questions to spend hours of time on.
Going through the bibles is a good way to study when you don't want to do another full PT, though. Even if you're reading about something that's not a question type you tend to have trouble with, it can help solidify concepts in your mind.
I also feel that the more RC sections I do, the easier they become. I really disliked that section at first, but I've really improved and don't find it particularly taxing anymore.
And with LG... I used to have timing issues with it, so I feel like I really have to keep doing timed LG sections on an almost daily basis to be sure that I'm not falling back into old habits or losing whatever it is that has caused me to speed up (better diagramming and inferences, I guess).