In general, if I buy plugs from a dealership which looks up which plugs I need based on my cars VIN or license number, do I need to worry about checking the plugs gap, or can I just assume they will be properly gapped out of the box?
After seeing Paul's answer, I just saw this on the NGK website when clicking on the Gap link:
A spark plugs’ tip temperature and the voltage necessary to fire the plug are directly affected by the gap setting.
Most manufacturers set the gap from the factory for that plugs most popular application. Unfortunately, that plug may have hundreds of applications from automobiles to golf carts. Setting the gap for your particular engine is important as insufficient spark plug gap can cause pre-ignition, detonation and even engine damage.
Whereas too much gap can result in a higher rate of misfires, loss of power, plug fouling and poor fuel economy. Even if the preset gap is supposed to match your motor, it is always best to physically check that the gap is adjusted properly for your motor prior to installation.