I made a battery charger last year - I put the fuse in the positive lead. Electrically speaking, it doesn't really matter whether the fuse is in the postive or negative. Putting it in the positive is only a convention, and electron theory is not of any importance in this matter.
However, when we start thinking about the mains circuits, in our house, it is important that ONLY active (hot) cables are fused. If a neutral had a fuse in it, and that fuse blew, one side of the fuse would become live. This creates a safety hazard, if someone touches the fuse terminals. Getting electrocuted is NOT a good thing!!!! :twisted: