"Do I need to have the car hooked up to the Dealer's computer to check for idle/fuel ratio/ECU timing/etc..."

For a '98, the computer is not used. A scan tool is used to check codes and usually only IF there's a specific problem or complaint. A tech should check for codes present but there's no guaranty. Fuel mixture, idle speed, timing are not adjustable nor do they normally ever change (unless there's a problem) so no one would need to check those items during basic maintenance.