I think alot of people believe that guidelines and a well set up grid is the same thing...
its not!!! (just alot of people having no idea what they are doin)
a good grid should define your type size, line length,hierarchy, dynamic movement, etc. A good grid should be thought about before you do anything. It should be based on document size and how the typography and images are gonna be used. In print, margin sizes and gutter sizes are all apart of grid development. To add tension and fluidity.
Alot of people here just regurgitate what they hear from other people without the appropriate research and experience.
Its a very good tool if done correctly.