Just seconding the comment above to the effect that where different materials (including what's *behind* the tile) meet, or even were similar materials are subject to differing change in temperature, unless you can provide some sort of mechanical expansion joint caulk is more durable that grout.
For this reason grout manufacturers make special color-matched caulks to match their grouts, available in both "sanded" and "non-sanded" forms to match grout texture, and custom mixed colors are also available.
In my experience the caulk to grout color/texture match is seldom exact, but it's been my observation, confirmed in conversation with experienced tile setters and contractors, that there are just some applications (such as joints between floor tiles and cove tiles or were corners meet at an exterior wall) were grout frequently fails, and that in these locations the best choice is to caulk with the understanding that ever decade or so you may need to do so again - at least the old caulk is easier to remove than grout.
FWIW, I've found Michael Byrn's "Setting Tile"
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to be an excellent resource on such questions.
He's a bit "old fashioned" is some of his techniques, but he got his start working for a company that repaired work done by others, so he's very aware of a number of potential tiling "pitfalls' that many not become apparent for months or even years after initial installation, and the book has hundreds of photographs and illustrators that clearly convey his approach to preventing such problems.
Michael Thomas Paragon Home Inspections, LLC Chicago, IL mdtATparagoninspectsDOTcom
8four7-475-5668