What sort of battery size are you thinking about ? It makes a big difference.
Small batteries with insulated faston terminals aren't too much of a problem - but again I'd suggest following the earth first off, earth last on described below. But as the batteries get bigger, the risks get higher. In the past I've worked with big batteries - I'm no lightweight, when one of the cells would have needed several of me to tip the scales in my favour - you definitely treat those with respect.
The bit about disconnecting negative first possibly comes from the automotive world where vehicles have been negative earth for many decades. The idea is, if you short the negative terminal to earth while undoing it (e.g. you put a spanner between terminal and bodywork) then there is no voltage (you are just going in parallel with the earth strap) and it won't cause a spark. Once the negative terminal is off, then you can safely remove the positive terminal as there's no circuit if you touch the spanner on anything (other than the battery negative which is generally hard to do on an automotive battery). Refitting is the reverse - make the earth terminal last.
Applying the same logic to a positive earth battery would mean undoing the positive terminal first - on the basis that it won't make sparks if you short the spanner to chassis. After that, you've broken the circuit for shorting another terminal to chassis and making sparks.
But then you get into all sorts of questions.
What system voltage is present ? Depending on the battery voltage, you may well get to the stage where some terminals are not safe to touch - either while touching something earthed or another battery terminal.
Is battery "earth" the same as the enclosure "earth" - that can't be assumed in all systems ?
And if it is, is that an easy terminal to get to ?
Have you got suitable tools - in the naval world they have fully insulated tools for working on big batteries, and I assume other industries will have similar. Dropping something non-insulated across the terminals of something like a submarine or BT phone exchange battery will (at the very least) mean that's the last you see of the tool. I recall a friend who worked for BT at the time telling me how a contractor dropped a pry-bar (what I'd call a jemmy) across the busbars in an exchange. It sparked as it connected, got too hot to grab and pull off before he could get to it, started glowing ... red ... yellow ... and finally dripped on the floor. For good measure, BT charged him (or his employer) for recharging the battery !
And then some systems are just "poorly designed". At a previous employer, we bought a UPS that used two strings of 10off 12V blocks, and IIRC 110AHr - i.e. 120V 220AHr nominal in total. The cells fitted onto several levels in a cabinet, and you had to start at the bottom, fit one connecting cable (which went to an isolator at the top of the cabinet) and some blocks, the inter-block links, and the link to the next block on the next layer. Then you fitted a metal tray on it's supports, and completed the first string - so now there was 120V across the battery side of the isolator.
You then fitted another tray before starting the second string. But, even though they had used a 3 pole isolator, they only used 2 poles of it. So you had to undo the bolt in one terminal, fit the end cable for the second string to it, and refit it - then build up the second string in the same way. Finally, you have a free cable which is now at 120V DC (nominal) relative to other bits, and have to fit it onto the same bolt as another cable that comes up from the depths of the cabinet. I "wasn't impressed" with that design - who designs a system that forces you to be waving a free lead about, and indeed grapple with two of them, when you've 120V just waiting to make your day rather memorable ?
