Everet said recently somewhere that the coils rarely ever go bad and being that they are $50-60 some odd dollars a piece? I'd spend that $100-120 elsewhere rather than on preventative maintenance on a part that's low risk to failure.
Read somewhere that if your driving habit is long distance hwy, your best bet is cold plugs to eleviate heat. If you were doing short trips, you wanted to use hot plugs for smoother fire in colder conditions.. My memory is fuzzy on this though. Something that I should consider doing is some new (hot) plugs. I think I went a step colder back when I used to drive long distance. I just drive mostly in town now. Could explain my rough idle..