I have tiled my own floors, ran base, wood and tile in one room or another. I have also flushed toilets and had them overflow. The construction of wood framed homes inherently allows for water which naturally seeks the lowest point will find the gaps at the plate line to the underlayment, (and tile!!) all around the perimeter of any bath. To me that has a lot of space to hold a lot of potentially unsavory water, if you want to call it that. Although the expansion and contraction of materials that make up a flooring system in a wood framed house is very small, I have run my tile close enough to the plate, when the drywall is cut high enough. When the drywall does not allow any room to tuck it under the cut edge I run it close enough to cover the gap with the base, as it should be done anyway. The point I will make, in either situation, I fill the gap with backer rod if it needs it, pushed just below the surface, and place a good grade of caulk over all of it to seal the floor to the wall. I cove the caulk into the corner so the bottom of the drywall is covered in caulk, that way it will not soak up any thing on the floor. To set the wood base, I plane or table saw the lower bottom corner off to make room for the caulk cove that is in place.
This may be a little over the top after all of that, but I use caulk set on the bottom edge of my base that I set to seal the base to the floor. This is applied to the base after it is cut and tried in place. I remove it, add caulk, set it and nail with pressure, pushing down.
I will not use grout anymore. I did on one job and the tile cracked because of the minor movement of framing I mentioned earlier.