How Important is it to do the 90,000 mile service...i went to take my car to mechanic shop and he says its not all that important, he told me to come back at 105,000 miles...plus i just got an oil change and rotated my tires, so all they have to do is a bunch of checks...should i pay a couple hundred dollars 4 them to check things? Also my problem is that i am getting about 20-40 less miles per Tank of gas. So i guess that means spark plugs need to be replaced?