I am installing a new tile shower using a Kerdi kit for the first time based in part on recommendations from this site. I am ready to lay the tile. Several sources I have been looking at show screwing a board to the wall and tiling the walls above that point. Then the floor is tiled and the bottom row of tile is installed so that it sits on the floor tile.

It seems like it would defeat the purpose of the kerdi membrane to do this. When you use the kerdi membrane, should one just start from the floor? Any guidance would be really appreciated. This remodel has been dragging on and I'm ready to get it tiled and be done with it.

I would not run any screws through Kerdi since that would defeat its purpose. When I did our shower I started with the drain and floor, then (I think) the seams, and finally the walls and mixer valve/shower head openings.

I believe the use of a ledger depends on each particular application. I believe the purpose is so you leave a gap at the floor/wall transition to be caulked instead of grouted. If you were to just stack the first course of tile on top of the floor tile, you obviously wouldn't have that gap to caulk and wouldn't have any room for movement/expansion without cracking your grout at that transition. If you try to start with leaving a gap, if the tiles you're using are too heavy, you risk the weight of the subsequent courses pushing the first course down the wall and closing the gap you started with. By using a ledger screwed into the studs, the ledger supports the weight of all the tile above it. Then you just remove the ledger, patch the screw holes with the Kerdi Fix sealant, and install the bottom course.

Disclaimer: This is my understanding from all the research I've done, not direct experience. I'm almost to the point you are on my first bathroom project (have to finish hanging the Kerdi this weekend). Others with actual direct experience may say something different.

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I believe the purpose is so you leave a gap at the floor/wall transition to be caulked instead of grouted. If you were to just stack the first course of tile on top of the floor tile, you obviously wouldn't have that gap to caulk and wouldn't have any room for movement/expansion without cracking your grout at that transition.

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This is generally correct, but the reason you caulk the bottom instead of grouting is so you don't wick moisture into the gap. I don't know much about the scope of your actual project, so your mileage may vary. If you are using Kerdi all the way to the floor for a shower stall application, you could consider wrapping about 3" of it under the floor tiles so the a gap isn't really needed. When I did my project, I tiled from the tub deck to the ceiling and used a piece of lattice mould between the tub and tile to give me a 1/4" gap for caulking.

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I believe the purpose is so you leave a gap at the floor/wall transition to be caulked instead of grouted. If you were to just stack the first course of tile on top of the floor tile, you obviously wouldn't have that gap to caulk and wouldn't have any room for movement/expansion without cracking your grout at that transition.

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This is generally correct, but the reason you caulk the bottom instead of grouting is so you don't wick moisture into the gap. I don't know much about the scope of your actual project, so your mileage may vary. If you are using Kerdi all the way to the floor for a shower stall application, you could consider wrapping about 3" of it under the floor tiles so the a gap isn't really needed. When I did my project, I tiled from the tub deck to the ceiling and used a piece of lattice mould between the tub and tile to give me a 1/4" gap for caulking.

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My application is for a shower stall and the floor has kerdi completely covering the base all the way to the drain. My thought is that the ledger board would primarily allow me to make sure that the grout lines stay straight and level and would allow me to make sure that the top row ( I'm going all the way tot he ceiling) is a not just a sliver of tile.

One idea I found while Googling was to use a ledger board, but to use 2 X 4 pieces to hold it up and shims to level it. The simplest idea was then to use double sided foam tape to hold it in place. Then the kerdi would remain intact. I don't really know how much weight the ledger board winds up holding and how well the tape would keep it in place.