Different types of software require different types of planning. If you’re working on your own blog, for instance, the stakes are a lot lower than if you’re working on, say, an automated flight control system.
If you’re working on one of those high-stakes projects, your planning should be much more thorough & much less iterative. Your development is much more likely to follow a waterfall approach. But on your blog, or even something like an e-commerce site, you can (and arguably should) be a lot more iterative.
In either case, specifying more prerequisites upfront will help save time and effort in the long run, but it’s more important in more sequential projects. But, of course, your project’s balance between sequential and iterative will depend largely on what type of project it is and the risks involved.
One common rule of thumb is to plan to specify about 80 percent of the requirements up front…S. McConnell, Code Complete, second edition. Redmond (Washington): Microsoft Press, 2004.
Essentially, you’ll need to pick the right balance for your project. If it’s something more stable, where the requirements are well understood and unlikely to change, a more sequential approach is appropriate. But if the requirements may change, or the structure isn’t as well understood, then a more iterative approach will be more beneficial.
Software being what it is, iterative approaches are useful much more often than sequential approaches are.S. McConnell, Code Complete, second edition. Redmond (Washington): Microsoft Press, 2004.
For more details (as well as an analysis of the potential costs associated with each approach), check out this section of the book.