Sleeping Rule #1: Take pictures?

I read somewhere once that you shouldn't take pictures of people when they are sleeping because it violates an intensely private situation. The thinking being that if you are witnessing someone else sleeping they obviously trust you and you'd be violating that trust to take pictures of the drool spilling across their pillow.  I am undecided though. For only one reason - Jo falls asleep with me much less than she used to and I know one day she won't fall asleep with me at all. And I want to remember this perfection: