In general there are several different textual elements to consider in a design. If you're waiting on someone to type an essay (or 5 full blog entries) before you've even set up your font grids, then you'll be in trouble.

If you're working with specific copy, attention-grabbing copy, then the exact wording of that is on every bit the same level of detail as the rest of the design and it's pretty necessary to have a good rough draft of the wording before you start crafting the appearance.

At the end of the day though, I find myself using Lipsum all the time to very good effect.