I agree that Rockband almost certainly does render a whole frame before scanout. So, yes, there's necessary latency there between the input processing and game logic and everything and the output. Higher refresh mitigates that, but that's not really an option on the kinds of gaming machines Rockband runs on; hopefully it's all locked at 60Hz, because that's what TVs run on (unless maybe ew PAL, but ew), and all the machines have a 240p mode, if they're not cool enough to run 480p or better.

But modern displays add unnecessary latency on top of that. HDTV CRTs often did too. Input devices sometimes add latency too. It's really not too awful as long as it's consistent, real musical instruments have latency too (especially pipe organs! but organists manage to figure it out). It's not as much fun if you need to hit the note when it's visibly far from the 'goal' section, or if the judging becomes visible a long time after the note --- shortening the feedback loop is good.