How detailed your grid is and how strictly you adhere to it should be determined by your content and audience. Where consistency and predictability are paramount, as in long documents or reference materials, a strict grid is indispensable. On the other hand, if attracting and holding the attention of casual viewers is a goal, then serendipity and variety have their uses. For things like event notices, interactive experiences, and some online newsletters, a more flexible layout approach may be appropriate. Use contrast to create emphasis.
Contrasts are the tools of emphasis.
Even in a purely textual page, the patterns of blank space, gray body text, darker headings, and rules are used by viewers to see the page's organization before they begin to read. Remember that computer screens are a low-resolution medium. A simpler, cleaner look may be more powerful than a cluttered one. Use judgment in applying new formatting capabilities. Just because an effect is possible doesn't mean it's effective on a specific page.