How do I hide rulers on startup for Pages?
Get a document set up exactly the way you like it, with whatever showing or not-showing that you want. Save it as a template (File: Save as Template).

If you have the template picker load on startup, then just pick that template.

If you don't, then go into Preferences, and choose that template as the template to load by default (For new documents: Use template…). 

These templates can also include custom styles, canned text, etc.