Hi Brett--
I don't think there's a way to control this on a simple document-wide scale. But if you generate your own PDF bookmarks instead of letting Styler (or FOSI) do it, you can control it at the individual bookmark level, e.g. you could have the top level expanded to show the chapters, but each chapter collapsed, or the first chapter fully expanded, and the rest collapsed, or whatever you want.
The Help Center contains information about the markup to generate in order to make this work. The instructions are written for FOSI use, but they also work using Styler via gentext. In the Help Center, search for "creating pdf bookmarks using fosi" and it will take you to the instructions. In the markup sample they show, notice that you can specify, for an individual bookmark, a "state" attribute, e.g.
<atidmd:bookmark state="closed">
<atidmd:title>List of Figures</atidmd:title>
...
</atidmd:bookmark>
to control whether it's open or closed on startup.
--Clay