Funny one...
I learned in college (still have the writer's guide) that there are two major styles - Modern Language Association (MLA) and American Psychological Association (APA). APA is used for psychology, social sciences, anthropology & business. For APA single spacing after periods was normal (although dual spacing was allowed) and for MLA dual spacing was normal.
As an engineer I have always double spaced. Don't think I can/will ever change.
Also, I have way too many things to worry about. Especially the grammar police and especially the grammar police on the internet...