Spark plugs..

[04FX4]

Ok so my 04F150 FX4 is going in on Tuesday to have the plugs changed. I have 35,000KHM and Ford says to change them at 100,000K. I called the dealer and he said the longer the sit the harder it will be to remove them and more chance they may break.I can understand that but after they install them when is the next time I will need to change them. I was going to do it myself, but for the $211 labour for the tune up its not worth the stress of me doing it.So what are all of you doing how often are you changing them, so that there is no major problems???????