Having had a home where the previous owner used silicone along the meeting walls of a shower stall, I hesitate to do the same thing. What that produced was soaked walls with mildew that needed an entire replacement. 

My current home also has a stall shower. Minor cracks along the meeting walls. So I used Custom Building Products Simple Fix grout repair product along the meeting walls and around the bathroom where the walls meet the floor. All joints were previously grouted where wall tiles meet the floor tiles (an old house). This grout manufacturer recommended  removal of the old grout and replace with silicone, and did not like the idea of silicone over the grout. But this is not practical as the old grout is very hard and I would surely crack tiles. I was also cautioned that grout sealant ( have TileLab grout sealant that’s a few years old) would not penetrate their material well and advised not to use silicone over the grout.  Even their professional silicone.

So my first thought was that silicone over the grout seemed like a good idea to me if the materials are well prepared first. I doubted that was done on my previous home.

On second thought,  I’ve become wary that if the silicone should become loosened over time, or otherwise permit moisture under it’s surface, it may just permit water to soak the grout, and I don’t want a repeat of my first experience. 

So the untreated grout will at least have opportunity if wet to dry out.

Any useful input would be appreciated.