I prefer using shrink tube. You place it over the wire before you solder the two ends together. You want to make sure you have it far enough from the heat while you are soldering otherwise it shrinks where it is and doesn't cover the splice. I feel other methods are acceptable when you are stuck in the boonies and you are left with little or no choice.
Electrical fires can occur when a positive line shorts to ground and the line is not properly protected for the current that it is being subjected to when shorted.