I normally buy guides with my games if they are available (or not super rare) and one of the things I like to do before I start up a game is look through the guide and instruction booklets to get myself a bit more immersed in the world I am going to be playing in and giving me a little more background or flavor text before I start. I think physical guides are the ultimate companion pieces to games, and I like to get both if I can.
As for guides in general, I think it's a good idea in general to at least take a glance at what you will need before you start a game to save you the headache of needing to invest another separate play through if possible. I would compare to getting something you need to build out of a box with dozens of little pieces, parts, and things that need to be completed in order to finish. Some people don't believe in using instructions when it comes to building these things, and some people like to work in a mess around them. I am one of those people who looks for those instructions first, and likes to separate those parts into piles by step, and go from there. My standpoint at games is fairly similar in it's approach, and normally requires equal preparation.
Time is the one commodity that can never be replaced, so I place a fairly high value on what I do with it. Guides are one of the best tools for maximizing ones efficiency when it comes to time, and should be used by anyone who has it in short supply.