I always take the neg off first as its easiest to make a clean break on doing so (what you dont want is it arcing as you are faffing about trying to wiggle it out), but it doesnt actually matter which, both are breaking the circuit anyway, obviously with the live though you need to be careful not to ark out, as if you touch the ecu case or similar with the spanner you are causing a short, so for that reason as well, neg first makes more sense, but if you are careful either is fine.