Use guidelines while documenting installation and configuration tasks and other tasks that describe how to use features within a product.
Include screenshots when screenshots:
Help reassure the user that where they are in the UI is the right place to be
Help call attention to a specific area of a complex UI
Support an example that is hard to visualize otherwise, for example, setting up a configuration that is not quite straight forward
Where they add value, documentation maintenance and localization might be time-consuming and costly. However, just as you would with words, omit the obvious and whatever does not add value.
Strive for an additive approach (add screenshots when the words seem inadequate) over a subtractive approach (do not include screenshots if they seem superfluous). In other words, be more open to screenshots in the future, but understand that they have to work in the document, not just be there because they can.