I don't know how important it is with modern electronic ignitions, but bad condensers (the giant metal-shelled capacitor connected to the coil) have been known to cause big problems on old points ignition cars when they're missing or wrong sized. I always put a fresh one on with a coil. I've never experienced a mismatch problem myself, but I don't throw out the old one just in case, I just put it in the glove box.
I've done well with Bosch Platinum plugs and with good old AC copper plugs in the AMC engines. In Subarus though, I've gained 3-5 mpg going to NGK plugs. I believe there's some voodoo in plugs where they are matched better to certain engines.