I do both. For the most part, I read line by line, but if it gets to certain types of dialogue or exposition, I just skip through and look for key words. I usually will always skip music lyrics and just assume the emotion or other reactions to be valid without having read the lyric printed. This was a thing in Tolkien, and a few other books out of the 80's or earlier. I haven't seen it much at all of late. I also skip the info-dumping parts, where it breaks the flow of the story to give some background piece that justifies some other piece in what was just said.
The worst piece I chose not to read through that I instead skimmed through was John Galt's speech near the end of Atlas Shrugged. Afterwards I was glad that I had skimmed through it as I had a problem with it from a structural standpoint. 40+ pages of written word does not translate into 45 minutes worth of speech. It's quite a bit more by a few hours. I ended up skimming through it though because I was already familiar with what was being said from the point of view of John Galt, through previous school work, reading synopsi, familiarity with the subjects being talked about by Ayn Rand and being told by others what it consisted of.
But otherwise, I read line by line.