It won't hurt, it may help, may not.
Some say pull them early to reduce the chance of the plug seizing and making it harder to get out if you wait the full mileage interval. To prevent it, you could pull them early, inspect, and reinstall, but if going to that trouble, just replace them IMO.
You also have the issue of even though the plug may still be "good" and fire just fine, it can cause poorer mileage and performance.
If they are easy to get to, why not though.
I am debating doing the Scion's plugs @ 50,000 miles (still 2+ years at current driving rate) because of the timeframe.
Originally Posted By: oil_film_movies
Plugs: Yes iridiums can easily go 100,000 miles +.
Since 1995, cars have had OBDII software & sensors which throw codes when misfire is detected, and it actually identifies which cylinder is misfiring.
http://www.underhoodservice.com/common-causes-misfire-codes/ if you want to read some more.
Practically, as long as all the sensors & pumps are working properly and the spark plugs are still working, the cylinder will not misfire. Therefore, I say wait until your OBDII detects a misfire before replacing your sparkplugs. Even then, if you get the codes (MIL, or Check Engine Light), it may be a plugged fuel injector causing it, so you can even try Techron gas additive at first to clear the code. In any event, you don't have to do anything on cars these days until the OBDII light comes on!
I agree with you that differential and transmission fluids should be changed every, I'd say, 75,000 miles, just to get the metal particles out. The fluid chemistry itself holds up OK.
You do not just wait for the CEL to come on. If you notice issues, there can be things going on that do not set the CEL.
Misfires take a certain number of occurrences to set the light, or even a pending code. Waiting for damage to occur before repair is not a good idea. Is it not better to replace a part before failure?