For me it is natural to say that the square root of a non-negative real number is non-negative. That is the common way to define "square root" in Sweden. In basic American text-books I have noticed that the square root might be defined as both positive or negative followed by a definition of the non-negative root as the "principal" square root. I am interested to know more about how you from all over the world define the word "square root".
I suppose that all of you define the symbolic expression Va (I mean \sqrt a) as a non-negative number. But I am not sure. In one of the books I studied I found V9=3D+/-3. For me that is wrong and I would give the student a square root sign (error symbol) in the margin.