My Sedona recently rolled over 80k miles. At 100k miles, the OM recommends replacing the iridium plugs. On this motor, like many other transverse V6s, the rear plugs are a bear to get at. Should I just suck it up and tear into it at 100k, or wait til I get a stumble in the idle? Do plugs degrade slowly over time? Or are they either working properly or not? I've heard of many people going well over 100k on iridium plugs with no noticeable change in fuel economy or performance.