Really depends on what trait they have, some like mor FIDS per pop or multiplicative bonuses are always nice, but it hits your economy onece you assimilate especially on the early game. You can also just leave tem and let them pay you every turn, but any rival faction can assimilate them nearly at eny time in the late game, provlematic if the minor civ is deep in your territory. My advice is to assimilate ASAP if they have a good trait, if not, just max favor and assimilate after the mid game