I've tested 3 methods of deleting...

1. Go to Photos, slide out the left menu, select "On Device" and delete a photo -- Seems to remain in Google+ but is gone from phone. I can still view it in the gallery if I select "Photos."

2. Go to Photos, look at the default view (which seems to be Google+ view), delete photo, am prompted that I understand I'm deleting everywhere. Photo disappears from Google+ and from phone.

3. Go to Gallery, select photo, delete it. Photo seems to delete from device but remain in Google+.

So it seems method 1 & 3 will remove it from the phone (freeing up storage space) but keep it on Google+. I'm going to keep an eye on Google+ to make sure things just aren't delayed in being deleted. (But given that the picture disappeared in Google+ when I tried method 2, I think I have my answer.)