If you have a lot of shapes and you want to have all them them have the text, here’s a quick tip. Copy and Paste can do this for you.
The secret to setting the text is to copy text from some source – for example from Notepad or from the text of another Visio shape, then select all the shapes you want to change. Then simply use the Paste command.
To clear the text on multiple shapes, is a little trickier because you can’t copy an empty text sequence, instead just copy a single space from any source and then paste it onto the Shapes.
Here’s a quick screencast that illustrates exactly how to do this using Visio 2007