Great job maintaining your car! GM recommends very long "tune-up" intervals of 100,000 miles or so. Don't trust them! Most consumers find out they need spark plugs when the computer turns on the Check Engine light and ignition misfires occur. By then, other damage may have resulted like burnt (and expensive) ignition coils!
It's better to prevent that by replacing the spark plugs at 60,000-80,000 miles, depending on climate area of the country and type of driving. Don't skimp on these spark plugs. Use the recommended application by your favorite spark plug manufacturer, usually platinum electrode type.
Jim O'Neill - Chino Autotech Inc., Auto Club SoCal AAR
SAE Int'l SoCal Section Board of Governors,
Vice Chair, Service Technology Programs
NASTF Service Information/Communications Committees
Auto Value/Bumper-To-Bumper CSC Nat'l Council
ASE CMAT-L1, CA. Smog Check Technician
AMI AAM Accredited Automotive Manager