If you are strictly steeping, and not partial mashing, then that would be only crystals and dark stuff like roasted, chocolate, black, etc., that don't need mashing, correct? Perhaps if steeped too hot they leach bitterness and/or tannins, similar to sparging too hot. I usually see 150-155F recommended for steeping, which also works for mini-mashing if you wanted to add some Munich or Vienna or base for character.