I have a ridiculous amount of fabric, thread, and tools on hand. Yet I often need more of something for a particular quilt. I plan, plan, plan, but the plan can change during construction. I usually work from "my version" of a pattern, meaning that I either see something that I like and work up something similar after putting it in EQ8, or I start with a published pattern and still put it in EQ8, and make changes. Even when working from a kit, I almost always make design changes, fabric substitutions, and use techniques that are not in the pattern.
That said, I'm about to start two quilts that don't fit my usual modus operandi. One is the Labyrinth Walk, for which I will follow the pattern exactly. (Well, at least until I get to the border, and then I might add some more to make it a bigger quilt.) And the other is a quilt that I have in my head that won't look like anything I've done in the past, won't be thoroughly planned, and just might be a total disaster.