I am building a steam shower, Kerdi all around. My question is the use of sanded caulk for all the change of plane joints. I have heard it will just melt. Is a Siliconized Latex sanded caulk appropriate for these joints?

According to TCNA guidelines, you need 100% silicon for that application.

However, in a Kerdi shower, I grout everything, including changes in plane. I've never had a problem with it cracking. I'm not a fan of any caulk or silicon in a shower due to the fact that it's a maintenance issue down the road, and will eventually harbor mold.

That's just my personal preference, though.

What is a change of plane in a steam shower, are you kidding me, we always grout out all work except for the curb to wall and later maybe the pan tile to the wall if needed.