I think this was one of the few that didn't make it to the detail.dtl config options.
You can change the default text style in the session but I am not sure if it saves it outside the current drawing, or even if it remains with the current drawing.
Annotation>Format>Manage Text Styles>"default">Modify>uncheck line spacing and enter 1.0.
Subsequent text will be created with line spacing at 1.0 and "default" unchecked in the annotation style dialog for the newly created annotation features.