Actually you should use a silicone dielectric grease such as Motorcraft XG-3-A or equivalent.
Yeah, but not everyone just happens to have such as frequently laying about as WD40 which will provide for the easing of spark plug boot and terminal installation just fine.
Now, if this were for ones' off-road vehicle which you were planning to submerge underwater, well............I probably would work a little harder at protecting and sealing things up better!
Scott.