I’m a “snap to guides always on” type of person, and I’m pretty sure Acorn is not remembering my setting between files, or between work sessions. It’d be great if this could be addressed in the future.
You can turn them on for all newly-opened documents in the Guides section of Acorn’s preferences.
If you save an image in the .acorn file format, the guide settings will persist between opens, so you might need to re-enable it for those individual files. If you’re working with non-acorn files (PNG, TIFF, JPEG, etc.) then the preference will be used for all of those.