Flooring Tile - Help-Gap between cement board and wall

I would like to have a question:
When I place the cement board on subfloor I leave a gap of 1/4 inch between cement board and the wall, then:

1. Do I leave this gap open or fill it with thinset as I thinset the
cement board on top of subfloor ?
2. After I set the tiles on top of cement board,
Do I leave this gap open or fill it with grout ?

Any advises would be very appreciated.
Thanks in advance.

First-time tile installer

The gap is left open, or backer rod inserted and caulked with sanded tile caulking, to match your grout, or trim out with baseboards or ¼ round. Never set tight to the baseboards!

Cracked, or heaved tile and grout can happen later, if you do fill it. :eek:

Take the base off it will make for a better looking job in the end ,then you can leave a 3/16 or so joint under the base and not have to worry about what it is going to look like when you grout to the base :cool: