Tiling an internal corner

What is the best way to tile into an inner corner of a shower cubicle - cutting the tiles so that on one wall they almost touch the adjacent wall, and then cutting the tiles for the other wall to leave a 3mm gap to the first lot of tiles, cutting them so that the tiles on both walls are the same distance from the corner, leaving at least 3mm between the two sets of tiles, or mitring the inner tile edges?
Is it best to grout or silicone fill the corner gap? I want to avoid the tiles being forced off due to slight movement in the walls, or to water penetrating the join.
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