Since you are joining two different materials together you should use caulk in the space where the floor tile and tub meet.
Gap between shower floor and wall tile.
Another option would be to fill the gap with grout.
And after installing the floor tile you can install that final or first row of wall tile.
Fill gaps larger than 1 8 inch.
Plus a 1 16 gap allows the caulk to properly fill the space and waterproof the transition between the tile and tub or shower.
Some like to use a ledger or other space saver to indicate where your missing row should be.
If your gap is smaller than 1 8 inch in width use regular caulk.
Similar to the gap that can exist between your wall tile and tub you may have a space between the floor and your bathtub.
Leaving a 1 16 gap between the tub and tile will prevent the tile from cracking when the tub expands.
If you have tile on the wall adjoining the tub select a caulk that matches the color of the grout.
But where two tiled shower walls meet or where a shower wall meets the floor called change of.
This gap may widen over time as your home shifts.
However due to different rate walls and floors expand grout will eventually crack again leaving you with the new hairline crack.
As with any other gap near your bathtub you ll want to close the opening to prevent any possible water damage or the onset of mold and mildew.
In addition leave a 1 16 to 1 8 expansion joint between the main wall tile and shower floor tile or adjacent wall tile.
Soak a sponge in water and wet the tile with it.
Place the 1 16 horseshoe shim on tub and measure up to the laser level.
Grout the gaps between the wall tiles take out the spacers and prepare the sanded grout mixture.
Solution would be to partially fill the gap about a half way with grout and to finish of read seal remaining with commercial grade 100 silicone caulk.
This joint should then be filled with 100 silicone to allow for expansion and contraction as well as providing a waterproof layer.
So to accommodate the flooring tile many like to leave a small gap between the last row of wall tile and floor.
Or fill the gap with grout.