I'll start the opposite opinion just for the heck of it.
Start with rebates, look at the plugs that is cheap to try.
Start with your car's ignition system, and use precious metal on both electrodes (double platinum, platinum on one iridium on the other, double iridium, etc) for waste spark, and single iridium or platinum on other types.
Pick the finest cheap to try precious metal plug, and the reduced ignition voltage will pay back in fuel economy and "performance" on butt dyno.
Check them on a regular basis to prevent using them for too long (30k for copper regular, 60k-90k for platinum or double platinum, 120k for iridium, etc). Check more often if you think something is wrong (i.e. insulator cracked, plug loosen out, etc).
Save your old OEM plugs, when in doubt try to put the old plugs back to see if problems are caused by the plugs.