It wouldn't really change much, to be honest. The odds your starting biomes would have boss spawners is pretty low. So realistically it wouldn't affect anything until you actually went looking for the bosses and found the nearest spawn location, in which case you'd have to hunt them down but wouldn't have to spawn them manually.

The only really serious effect would be with Eikthyr, which you would probably encounter extremely early, way before you could deal with it.