It depends on the specific design. If it's wordy and document-like (eg: http://www.w3.org/TR/html5/) then it may be better to bypass Photoshop and start writing the HTML/CSS, since Photoshop is ill-suited for mocking up those kinds of designs.
On the other hand, if it's more graphical (eg: http://www.ubuntu.com/desktop) then I'd agree that Photoshop is absolutely necessary.
And no, I still disagree slightly. It's not about the kind of design the final product is, it's about what function your role is fulfilling. Are you designing the way the people interact with the application, the way the application looks and feels, or both?
On the other hand, if it's more graphical (eg: http://www.ubuntu.com/desktop) then I'd agree that Photoshop is absolutely necessary.