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