Its super easy and super cheap job..so why not?
I replaced the original plugs in my 4.6L at 120k miles. The gap of the worn plugs was twice what is spec'ed new. It turned out that the wide gap would periodically cause the engine to flood on a cold start. Since the plug change this has not happened again. This is the first time I have had this happen in any vehicle I have owned. The 4.6L plugs have a very narrow center electrode that wears away quickly. They are probably toast by 100k miles and should be changed.
Between the plug change and a recent TPS replacement my vehicle idles, runs and shifts noticeably smoother.
Some people like to take them out more often to see if there is any indication of cylinder malfunction, or if they know they burn oil then deposits build up much faster, and in some cases (like my '98) it would be many years between the ~100K mi change and the next change at 200K mi, wouldn't want to risk them being seized in because they haven't been changed in a quarter century.
It wasn't anything serious. I could slightly feel the shifts occurring when mildly accelerating. After the TPS was replaced the shifts are smoother and less noticeable. The TPS completely failed on me before replacement. My guess is that before it failed it was probably sending voltage to the ECM that was slightly off and/or erratic which might have messed with the transmission module shifting gears to a mild extent.
