Grout vs caulk in shower
I know this issue has been discussed a lot, but I wanted to describe something a recommended contractor told me.
First, we have a 4x8 walk-in shower using porcelain tiles. The house is 5 years old and we live in the midwest. At a 1/2 dozen spots on the perimeter of the floor, on the joint where the walls meets the floor, the grout/caulk has turned black. I'm assuming this is mold or mildew. No matter how much we try to clean, it doesn't seem to get rid of it. So, our plan is to remove the grout/caulk and have someone re-do it. I have referred to it here as "grout/caulk" because it appears to be a clear caulk over the same grout that is used everywhere else in the shower. The shower seems to be in good shape. I don't see any cracks in any of the grout, either on the walls, floor, or the joint between the walls and floor. Structurally, it seems good. It's just that we have these black spots/streaks that are unpleasant to look at.
We called a plumber we have used and he recommended a contractor that specialized in tile shower repairs. When he gave us the estimate he said he had been doing this for like 20 years and can't understand why builders' contractors put caulk over grout. He stated that moisture inevitably gets behind tiles, and the caulk prevents the grout from "breathing", thus causing the mold. His advice was to remove the grout/caulk and re-grout. He would NOT put any caulk over the new grout. He gave us a quote for about $400.
At the time, that seemed like a lot of $$$, so we had another contractor look at it, someone from Angie's list. My wife was home at the time so I didn't get a chance to ask any questions. The quote they left said nothing about grout, only "re-caulk" and the quote was about $350.
Based on what I've read in this forum, it seems that the recommendation is to use caulk wherever there is a change in plane. So, it sounds like we should remove whatever we have there now and apply a caulk that has a similar color to the grout. But what about the concept of grout "breathing"? Why do some spots mold and others don't? I realize that what I thought was a lot of $$$, may not be so. I'm fine to spend the money as long as we're getting it done right. Can some expert help me to feel confident in the correct way to handle this?
Thank you very much.