I still think mockups are important, especially for explaining to clients the approach you took in regards to UX.
What I do in this situation is I actually code up a basic version. That way, they can see the interactions. Is it polished? No. But arguably a mock-up wouldn’t be either. My way isn’t the ‘best’ way. Just the method I prefer. I have found that for me specifically, they are an extra step that doesn’t necessarily improve the final product.
But… that’s not to say you shouldn’t wire-frame at all. I’m just saying I don’t use Photoshop or any other photo editor. @chrisburton does and that’s cool.
If mock-ups in Photoshop work for you, use ’em OP. But also make sure you understand the constraints that CSS might put on your design. Far too many designers aren’t totally familiar with how to implement code and what challenges that might involve. Granted, that’s becoming more rare but I still see it with friends’ stuff.