The maintenance schedule says to replace the spark plugs at 75k miles. I am at 78; and being better gas mileage than ever. 23.9 in around town driving. I don't want to mess this up. Should I just leave it alone till there is an issue?
I changed iridium spark plugs on another vehicle at 98k miles. Took out Denso and put in NGK iridiums. No change in fuel economy, performance etc but at the same time there were no issues prior to change. It's really up to you to change them. You should know your engine's condition best. In my case on the other car, I probably could have let it go for another 10-20k miles.
Changed out the plugs today, super easy job taking no more that 30 minutes. Just like Orangutan said above. It's very helpful if you have a 5/8 spark plug socket with the rubber insert to hold the plug while you lower it into the hole.
Original plugs still look pretty good even after 92k, amazing!
On my '04 Mazda3 w/2.3 engine I replaced them at 110K miles. My fear was they were locked in there as it seemed to take a lot of pressure before one moved. I talked to a Mazda tech and he said he'd never had a plug actually freeze in, so I just used a pipe extension on the socket wrench and they all came out OK.
I found my plug socket had a tendency to come off the socket wrench and stay on the new plug once installed, so a needle nose plier was necessary to remove the socket from in the plug hole, you can also just tape the socket on to prevent this. I gave each new plug thread a light dab of anti-sieze compound as future insurance. If you have an air source, it's also a good idea to give each plug hole a blast of air to remove any dust or debris before removing the old plugs so nothing falls into the engine when the plug is removed.
To be honest, I noticed no difference with the new plugs, but it's better to change them than to wait until they possibly stop working correctly, or they do get corroded in.