Common signs of failing spark plug wires include a decrease in power, acceleration, and fuel efficiency. In addition, the engine light coming on or visible damange to the cables can be signs of a failing wire.
It is similar to a spark plug wire but isn't visible. Over time, the rubber deteriorates and loses its insulation quality. The spark from the coil changes direction; instead of traveling through the boot, from the coil to the spark plug, it will shoot out the side of the bad boot and spark to the cylinder head.
It is not necessary to change the wires but it's a good idea. Here's the reason. If your spark plugs have been in the engine too long the component at the end of the plug get's oxidized over time from having such high voltage run across it to jump a gap and create a spark.
Check the gap on the new spark plug before installing it. A new plug will often come pre-gapped to match your specific engine, but use a spark plug gauge to verify that it matches the manufacturer's specifications.
Typically, the consensus among the mechanical engineer community is that a spark plug for a lawn mower will last up to 25 hours of use. Some users simply prefer to replace it every mowing season as a part of their yearly lawn mower maintenance.