i might go ahead and change them, the headers make it easy to get them out, and by clean i mean knock the suit off with a wire brush and wipe them down with a dry rag, no water or chemical

 

I'll second the 02 sensors. They went bad on my 03 focus. No CEL just went from 40mpg highway to 25, this was at 80k km so 50k miles

 

I'd change the plugs before they get bad to change out anyhow that way you don't eat your threads up from waiting longer.
I changed mine at 40k and they were pretty stiff well except for the one that came loose, I wouldn't of changed them otherwise but now I'm glad I did and I lubed em up with some anti seize that way I don't eat the threads up later either ha
I ended up paying $100 I think maybe $120 after taxes was probably cause I was in the middle of South Dakota though, still the most expensive plugs I'd ever bought.