Summon Rotation

doesn't actually affect the rotation that a /summon 'ed mob spawns in.

All 4 of these commands will summon a Creeper facing South no matter what even though 3 of them should spawn facing a different direction:

/summon Creeper ~ ~ ~ {Rotation:[90f,90f]}
/summon Creeper ~ ~ ~ {Rotation:[-180,0]}
/summon Creeper ~ ~ ~ {Rotation:[180.0,0.0]}
/summon Creeper