Bleach apparently won't kill mold (read this somewhere while researching/cleaning). When we had a recurrence of tile/caulk darkening in one small spot at the base of the MB tub surround, I scraped out the caulk in that spot, and then hit the space with vinegar, hydrogen peroxide with and without baking soda, not necessarily in that order. After cleaning it all off/out, I let it air dry really well, and then re-caulked, using the same siliconized latex caulk (that matched the grout). If it does happen again, we'll scrape out the caulk, and use 100% silicone caulk. We also wipe down the walls with a microfiber cloth after showering, especially where the tile meets the tub. I couldn't find a squeegee that I liked, and I think the cloth does a better job wiping away water droplets (which, with our moderately hard water, leaves ugly water spots on our shiny white tile), and does a better job sopping up the pool of water that would otherwise sit along the edge of the tub, at the base of the tile and caulk.

As for moisture permeation, even if the joint's NOT cracked, moisture can still get through. The caulking is there to provide for a moveable joint, not a watertight one. As for why more installers don't use caulking, it's because they don't know any better, are too lazy, or don't want to spend the extra money on some stupid customer.

I'm under the impression that if everything is done right...that all is nice and level, then nothing should crack..I also don't see how anything could crack... If only it were that easy. :-) Here's the problem-- different surface will move in opposition to each other. In this case, the countertops (which indirectly, are influenced by the floor), and the backsplash, which is attached to the wall. If that were the only problem, I could understand where maybe a thin joint would make the difference. However, there's another monkeywrench thrown into the works. Think about all the times you put bags of groceries up on top of the counter, or ANY heavy items (including your rump or knees when trying to reach up high). Also, think about times when you've allowed the dishwaasher door to close on its own, causing vibration in the countertop. How about the times you've bumped up against the cabinets or countertop , either from losing your balance, or shutting drawers, or for whatever reason-- all these things will cause movement at that joint, and the SLIGHTEST movement is enough to cause grout, which has no give whatsoever, to crack at that joint.

With the work being done a few weeks ago, there is nothing wrong with calling the installer who tiled and asking why grout was used in the changes of plan instead of caulk. Ask about the corner grout cracking. What if? Will remediation be offered? Caulking a shmear of caulk over a grout joint is not going to prevent the grout underneath from cracking if the grout is prone to cracking. Some corner grout never does crack. With your house being an "old 1060s", it may be nicely settled in. You may not have differential movement between the wall planes that leads to corner grout cracking. Grout sheds water. Caulk sheds water. Both are superficial things that "pretty up" tile installations. The real waterproofing should be behind the tile. Caulk needs some depth to it, if it's just a veneer over grout it may peel delaminate over time. Latex caulk versus silicone? Both will shed water fine, but 100% silicone will perform better in standing water than 100% latex. You'd be correct to make a phone call and to ask questions. And to get answers. Sometimes I'm of the mind that although best practices may not have been followed, second-best practices can still turn out fine. Meaning, you could leave the installed latex caulk until you have issues. You may never have issues. But if you do, then replace it. Same with the grout in the corners. Leave the grout in the corners until it cracks. Then replace it. When you talk to the installer, if he offers to come over and remove the grout and replace it with caulk? Sure, you could take him up on it. Or just get an assurance from him that if problems develop down the road, he'll swing by to make things right. It's best to get those assurances in writing. It's really what you feel most comfortable with. And how accommodating the installer is with regards to backing up his work.