At this time, you can't default the previewer to "print" in Atticus. We don't recommend this because when you're defaulted to "print," and working with the previewer open, it will render a new copy of your book with every change you make. Depending on what changes you're hoping to see we have a few recommendations on how to view your book in a print version.
Easily view quick changes:
If you want to see quick changes you can select "draft" mode when you select the "print" preview.
Final formatting and changes:
If you want to see larger changes you've made to your book we recommend using "Full book preview" to view them. We have an article that walks you through how to access it that you can find here: "How do I access Full Book Preview?"
We hope this helps!