Let's address the dangerous part first!The reason that the manuals (and most mechanics) say to disconnect the negative side first is that if/when the wrench you are using to loosen the clamp or bolt touches a metal part of the car, you won't short out the battery. Doing the same while removing the positive first would result in a direct short from positive to ground (zap/ka-boom!). For those who learned it the other way, now would be a good time to change! Those of you with Jags and other positive earth vehicles may continue to disconnect the positive first.

If you disconnect the positive, you could still short the battery by dropping a wrench across the positive post and any nearby metal part of the car, whereas with the negative cable off the only way to short the battery is by dropping the wrench on both top posts. Safer all the way around disconnecting the ground cable.

An interrupted circuit is an interrupted circuit, so a quick disconnect can go on either side of the battery, whichever is more convenient. That being said, the master switch on race cars is on the positive side and if wired correctly has a separate circuit for the alternator to completely shut off the car when it is running. If the vehicle is off, it doesn't matter, and I'm assuming that the cutoff is to prevent battery drain when sitting for a while, not to kill a screaming engine when the car is upside down!