I think the majority has to do with the complexity of the flavor. A simple fruit flavor (apple) would probably be good right out of the mailbox, where a more complex liquid (banana nut cinnamon whatever) would take a bit longer.
As for tobaccos, most NET liquids take some time to develop, whereas synthetic tobaccos seem to be good right out of the package.
As with the great flavor debate, steeping is just as subjective as taste. My method has always been to try everything right out of the mailbox and determine if it is something I like or if it should just be put away for awhile.