Let me state that I'm a big fan of some of the great bands of the 60's, and I've been known to listen to my share of Dire Straits and Little Feat as well.
At some point during a setup or tuning session, I'll most definitely pull out some rock music, but this is typically at the end of a session.
I listen to a wide range of divergent music, including 20th century string quartets, bluegrass, various types of jazz (Afro-Cuban, "free", gypsy, bebop, etc.), and my system setups cannot play musical favorites. A system should bring out the best in all genres (to the extent of it's capabilities). This is non-negotiable.
Even if my tastes were limited to rock music, I'd still own a few classical pieces (at a minimum) for system tuning.
There's an argument that goes, if you know the sound of Neil Young's 5E3 amplifier going into full meltdown, or Pete Townsend's "dimed" HiWatt stack (it doesn't need to go to eleven), then not only will it come through in a well-tuned hi-fi, but you can also tune your system with these recordings.
To keep this essay short, I don't want to get into the tonal aspect, other than to say that if you get it "right" with an acoustic recording, then you'll like your electric recordings even more.
There's more to the musical picture that you can't suss out by listening to rock music however, and it has to do with dynamic range. The problem lies in how compressed rock recordings are. Rock recordings won't challenge a system's ability to decode micro and macrodynamics.
You could argue that if the music is compressed and that's all that you listen to, then there's no problem. This is another case of not knowing better until you experience it. If you tune a system's dynamics with challenging acoustic music and you return to your Led Zeppelin, it will sound better. You'll gain more insight into Jimmy Page's picking technique and note attack and decay will take on more realism.
In yesterday's tuning session, our customer pulled out some old warhorses - favorites of his like Mark Knopfler and Emmylou Harris ("All the Road Running"), T-Bone Burnett's self-titled album, "Deadicated", and a few others.
Something was off, but I couldn't put my finger on it with these recordings (which I'm very familiar with). I always bring a few of my albums to these sessions - especially when I suspect that the genre isn't represented in my customer's collection.
I suggested we listen to the Original Cast Recording of West Side Story. The overture told the story in two minutes. Microdynmics were crushed, crescendos were flattened, and finger snaps sounded like anything but the sound produced by human flesh.
The specifics of what we tweaked isn't relevant to this conversation (stay tuned), but suffice it to say, we would have been chasing our tails if I hadn't brought a few records to the session.
Bonus #2. Having a few recordings you might not ordinarily listen to (and dialing your system in so that it honors them) might just introduce you to musical genres you might not ordinarily listen to.