I usually read it for 3 to 3:30 thorughly and then answer the questions. Of course, I go back to passage from time to time, especially when there is some question that address something specific into the passage.
But skimming of a passage - no, I've found this tecnique extremely uncomfortable for me. It could help for some other tests, but im GMAT, and especially LSAT, passages are brutal. They have mid-pasus that is real brain fryer. Twisted upside-down, you often cannot catch what is the cause and what is the consequence. I really haven't found skimming helpful - quite opposite.