If you opened Potion, it should be because you think it's very important to get a Familiar asap. So you should do the +cards and buy the Familiar. If buying the Familiar asap is not important, maybe you shouldn't have bought Potion. On its own, Steward is not a reason to ignore Familiar. But Familiar can play very differently on boards with strong trashing. Depending on the engine, sometimes it's better to build the engine first, then add Familiars. If you buy Potion, then Familiar on early turns, you're giving up a lot of tempo on the engine building, since neither are useful for anything other than giving curses.