how essential are grids to web design

So much web design work relies on establishing a grid and the constraints on that grid: ad sizes, display size, browser display area minus chrome, and so forth. Grids are, quite literally, everywhere. But learning how to effectively utilize grids-- without becoming a slave to them-- can make the difference between a competent layout and a great layout.

http://www.codinghorror.com/blog/archives/000877.html