I'll often stay logged in with the browser window open in the background if I want to do something else on the computer for a bit. But I always log out of everything when I walk away from the computer. That's just years of safe/secure computing practice that's become a habit. It's easier that way; I don't have to worry about "Did I do it or not?", because I always do wherever and whenever.

But as DocJohn said, it depends entirely on your situation.