To sum up, freestanding and clawfoot tubs can sit on top of a tiled floor, but it's not a good idea to tile under drop-in or alcove tubs.
Actually, the tile should NOT rest on anything. It should hang just a ~tad~ above the tub (flange or deck or whatever). Your distance picture looks pretty good and I agree that the first pic is a bit confusing to discern.
Plan to extend the tile 2 or 3 in. beyond the tub and leave at least a half tile along the wall if possible. Don't stop tile even with the end of the tub. This leaves the walls along the front of the tub vulnerable to water damage and doesn't look as finished as tile that extends to the floor.
Generally, the tile will extend past the edge of the tub one full tile including the necessary trim. Tile and trim can be installed using a trowel-able paste made from Portland cement mortar on a non-cured bed or dry set/latex modified Portland cement mortar on a cured bed.
White silicone caulk neatly applied would be your best bet. It'll contract and expand with the tub and the tile as they both do, fill the gap nicely, and be easy to maintain. If you want to get a little wild, use some black silicone caulk.
For a shower enclosure, extend the tile and the backerboard at least 6 inches above the showerhead. For a tub surround only, install the backerboard and tile at least 12 inches above the tub. For a high-end look, take the tile all the way to the ceiling.
Typically, you should use caulk in bathrooms due to its flexible, waterproof, and non-porous composition. Caulk is also easier to remove and replace when necessary, which will minimize damage to your fixtures. Keep in mind that sometimes a project will require both grout and caulking.
To sum up, freestanding and clawfoot tubs can sit on top of a tiled floor, but it's not a good idea to tile under drop-in or alcove tubs.
Tub surrounds over tile are the most cost-effective and simplest method to update your bathroom. Tub surrounds are your best bet if you're tired of the dull tiled wall around your tub and want to replace it without much mess or cost.
With placing the bathtub before the flooring and wall, you can better position the bathtub for protection and aesthetics. Without having to worry about damaging the flooring and wall, you can simply place the bathtub where you want and place the flooring and wall after to limit gaps between the two.
Install the Shower Tile
Once you have marked the tile layout, you can start tiling the shower wall. Start by placing the first row of the tiles at the shower's bottom and continue adding one row at a time. Utilize the centerline you established to place your first tile and work your way out to the corners.
As a tile installer and bathroom re-modeler, I recommend going to the ceiling if the conditions are right. There are a few reasons I encourage my clients to tile up to the ceiling: Tile is meant for a wet area. No one likes to see wet paint and drywall.
I usually go past a few inches. I also go past by a few inches at least. A lot of times I'll let the size of the tiles and the layout help me decide where to stop. I also like to bring it out past the tub and down to the floor.
The backer board should overlap the tub flange so the tile has a stable backing to attach to. This requires shimming the backer board out from the studs so the wall is plumb. There should be a 1/4" gap between the tub ledge and the backer board.
Freestanding tubs do traditionally sit on tiled floors, however it's not necessary. Wherever you place your tub, just make sure you can install the silicone base to keep the tub in place.
In the context of a 3-piece shower, caulking prevents water from seeping into the cracks where the walls meet the floor or other surfaces. Without this sealant, water can penetrate behind the walls, leading to potential damage, mold growth, and even structural issues.
Shower surrounds can be installed directly over securely fastened ceramic tile. Loose ceramic tiles and plastic tiles must be removed, and the walls will need to be sanded smooth before installation. In new installations, the enclosure should be applied over waterproof backerboard.
The tub will expand much faster than the tile which will put stress on anything non-flexible. Grout is extremely non-flexible and will crack and pulverize at that joint. Caulk remains flexible. And really, you want the 100% silicone stuff, not just any caulk.
Aesthetics & Placement
When you install the bathtub before the flooring, you can position the tub for better aesthetics and protection.
When it comes to durability, solid surface shower surround is the more durable option. It's resistant to chips and cracks and doesn't require as much maintenance as tile. Because solid surface has no grout, there is no cracking between the tiles causing water damage.
If you prefer a material that is easier to apply and remove, grout is the better choice. Caulk takes a steady hand to apply, and it can be difficult to remove.
Epoxy grout: Waterproof, durable, stain- and shrink-resistant. The best grout in shower walls, backsplashes and other surfaces susceptible to moisture or stains.