I use silicone in all corners, joints etc. I fit bathrooms as well so I also use it as an adhesive and to assist with water proofing when I secure sanitary items to the wall.
When installing or tiling around a shower tray I always use 2 seals.
1) Between the edge of the tray and the existing wall/surface to be tiled, then let that go off then tile...
2) Then the second goes to seal the shower tray to the tiles.
The way I look at this is that if water does get behind the grout (and I've put a few right where this has happened), or heaven forbid, there is a fault with the visable seal
, then the second will act as a backup.
If I am putting 6mm hardibacker onto existing walls with a bath or shower tray in situ (as some customers have wanted), I may indeed put 3 seals in place.
1) Against the existing wall
2) Against the hardibacker
3) Against the tiles
The great thing about silicone is that you can hide virtually every cut you make to fit an internal corner because we all plan out our jobs before we tile.
What I don't do is silicone against walls or surfaces that have yet to be painted, because you obviously can't paint onto silicone with any great success. The shower room I've just finished is due to get painted by the customer. Hence, as I'm going to be doing his other bathrooms I'll be on site to do this when he's finished painting.
I also advocate it for use between floors and walls, whether they be lino or tiles.
I have also used it on trims below raised shower trays.
Discussions on all aspects of tiling & installing tiles in the tiling forum.
in a nut shell where and when to silicone 
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