??? there is absolutly no electrical difference between disconnecting the +ve or -ve terminal first.
If I need to disconnect the battery to do any work on my car, I usually disconnect the negative terminal for the sole reason of safety. If I disconnect the Positive terminal, it leaves the positive terminal on the battery exposed, so if I accidently drop a spanner between the exposed positive battery terminal and any other metal surface in the engine bay, it will short out the battery and can damage it, or other components.. and at worst, cause the battery to quickly overheat and explode.
By removing the negative terminal, it leaves positive terminal covered with the plastic guard, but even if it didn't have any plastic guard, it also isolates the metal frame of the car from negative side of the battery, so dropping a spanner between the positive terminal and the metal of the car, won't cause a problem.