Fill gaps up to 1 4 inch in width with caulk.
Gap between tiles and wall.
Caulking is the best choice for this application.
The large format 18 inch tiles should have inch space in between.
This will give you say a perfect 3 8 caulk line with an irregular wall to counter gap.
If your gap is smaller than 1 8 inch in width use.
Two inch tiles are factory spaced and sheet mounted.
You can try and maybe nothing will happen but if it cracks you will have to dig it out.
Four inch tiles called 4 and a quarter in the tile trade feature built in self fitting nubs that give the tiles spacing between 1 16 and 1 8 inch wide.
Your tile person will take the largest gap and run a tape line using that as the thickness or the caulk.
Tile stores generally sell caulks that match the grout.
Therefore the spacing is predetermined and usually averages 3 16 to 1 4 inch.
Your walls and floors are pretty stiff but there can be movement between the two that will crack the grout.
The tile council of north america recommends at least a quarter inch gap between the tile floor and the walls.
Whenever tile has a right angle like that grout is prone to fail so caulk is used.
If you have tile on the wall adjoining the tub select a caulk that matches the color of the grout.
Any gap should never be greater than 1 8 inch.
Never fill this gap with grout because tile grout is not flexible.
Ceramic tile wall gap back of tile should rest on finished wall.
For smaller tiles is advisable to use small spacers.
Instead fill it with an elastic material capable of sustaining the movement of the floor and the movement of the wall.
When you install ceramic tile on the floor you must take care to create an expansion joint between the walls and the tiled floor.
Current trends are moving towards leaving much smaller grout lines between tiles.
Get photos of great finish details include photos with bid and contract.