I would put down a backer board of some sort. It goes by several names, but cement board is one generic version, I think. Hardi-backer is a brand name. When I did my bathroom floor, I screwed down quarter inch boards, and tiled over that. Check carefully to see that you don't end up with a finished floor height that interferes with doors. Also - you may have to extend the height of the toilet flange so that you get a good seal, as the wax ring may not be enough once you raise the floor level.
use underlayment for tile or vinyl flooring specially use for that surface to eliminate rough areas. Also it makes flooring even with adjoining rooms, The ply you are referring to the sub floor. The toilet flange gasket can make up the difference in height depending on buying number 1, 2, or 3. I would only use 1/4 underlayment.
Ceramic tile or vinyl tile? Ceramic tile you need a total substrate of no lees than 1 1/4. This means you need to add a minimum or 1/2 to make it sturdy enough for tile . And the very best ting to use ( for the average DIY) is 1/2 cement board. And this needs to be properly installed. This will comply to ATC and TCA tile codes.. If your using a vinyl tile you possible can go over it all w/o underlayment but I strongly suggest a 1/4 wood underlayment such as luan. This gives you a clean surface to bond to. Any questions you can e mail me through my avatar and check my qualifications there. GL
