I took 18 wither skeleton skulls to an end portal to kill the wither boss for nether stars. The first two bosses I summoned did not produce any blue skull projectiles and did nearly no damage to the surroundings. The third and following bosses did, however. This was all during the same gaming session, so I don't know what the difference between the first two and the subsequent bosses was. Attached screenshots showing the first boss without skulls anywhere and the fourth with skulls.