Floor Grout Keeps Cracking?

I have all tile floors, which I love, so easy to clean. I have the same issue in my living room; one grout line that goes down about 2/3s of the room, dead center, grout is cracked with a little that has fallen out. I asked my tile guy what to do, when he came to redo the tile in the bath. He says that generally it is because of settling of the building; not necessarily that you have foundation problems, but just the day-in-day-out movement of building.

He says I have 2 choices: 1. pull out grout and refill (which I have done several times without success as it returns) but this time make sure I have sandy grout and mix in a little silicone caulk for give and this may solve the problem, or 2.

do nothing since it is covered by my area rug. I have elected to do nothing since the above is a maybe solution, I live in Texas where we can have a rainy year followed by 2 drought years so our soil shifts and this will probably happen again.

My consolation is it is on a grout line; the actual tile is not cracking. Oh, and mine is under the area rug. If it showed, I may have a different attitude.