I have about 64,000 miles on my jeep. I change the oil at proper intervals, I use a one quart filter. When I back out of my drive I feel a "clunk" transmitted through the steering wheel. Its just barely noticeable.
Its not the ball joints as I checked those really closely. So I look at the axle u-joints and I could see a faint rust line around one of the caps on the cross.
I had them changed and both were bad. The grease was kind of rusty colored in several caps. Clunk is gone.
So today I changed my plugs and I am glad I did. Recommended gap on new plugs is .035" (with the type that have an adjustable gap). The ones I removed measured .055, .060, .065 and three were .070". My coils were working overtime.
So if you are thinking about letting them go until 90,000-100,000 miles don't do it. Change them or at least regap them. But if you are going to pull the coil pack and reinstall them with anti-seize and di-electric grease and torque the plugs and coil bolts, you may as well put in new plugs.
That's my story and I am sticking with it.