Yeah, design is a separate discipline from programming and has to be considered on it's own. It can't be boiled down to just a set of techniques (Photoshop techniques, in this case). It's important to learn design theory (what you'll get from those Peepcode play-by-plays or from some good design books) and the practical techniques (what you'll get from sites like http://psd.tutsplus.com).

I'd recommend tackling both design theory and Photoshop techniques, unless your only interest is in implementing other people's design work and you just need to know Photoshop techniques to translate that work from PSD to HTML/CSS.