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.