Hi Rob,
I always install the corner dishes at the same time I'm install the tiles. Use thinset mortar just as for the tiles. Sometimes when the job requires a standard 4x6 soap dish or towel bars that might get in the way when grouting, I wait and set them just after grouting, using unsanded grout which works great. Do not do this if you're grouting with sanded grout.
Jaz
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Tile 4 You..Troy,Michigan U.S.A.
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