You probably should unless the current rating of the wires are much greater than the highest current that might flow. On the battery side max current that might flow from an ordinary lead acid battery could be 500 amps or more. If the regulator fails that could get onto the solar panel wires. If you put enough current through a given wire it will get hot. Wires are allowed to warm a bit and that leads to their current rating under different standards. But you dont want a situation possible where enough current can flow in a wire to go past warm up to melting the insulation or worse red hot fire temperature. So you put a fuse in series with the wire that will burn out and open at a current level less than what makes the wire warm past its rating. When you realise how much power and heat generating capacity is in a 100ah 12v ordinary battery you realise definitely put fuses in! High currents without proper fusing could create hot wires.... Not good...ask Swissair.
