I always use the Silicon sandwich technique. If tiling a shower or bath I will Silicon first (any colour because you are not going to see it) then tile then Silicon again. As for corners, just the finishing touch is enough as its a movement joint. I normally use Ardex Silicon and had no issues.
View the thread, titled "Best mould resistant Silicon" which is posted in Tile Adhesive and Grout Advice on Tilers Forums.
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