What is "proper" changes all the time, GorGor. Some people hold out for the way they learned it from their keyboard teachers in high school. Others will embrace the new and argue eloquently for why their way is better. They can argue all they like. Proper, in this case, is 100% personal preference.

Some computer programs don't care and will suck out that extra space even if you type it in. I have put two spaces after every period in this comment, but I believe when the comment shoots up onto the page, those spaces will disappear.

Publishers all have their own style guides. People who want to be published will submit works according to the preference of their publishers.

My students can do what they want regarding spacing after periods, providing they are consistent.

Right-margin justification screws up all the spacing anyway. Perhaps I'll write about that in due course. Perhaps I'll write about the myth of what is "proper" and "no longer proper."

If all you have is 140 characters of text on your cell phone, you may not have the luxury of two spaces after your periods.