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"
(Amazon.com product link shortened)
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.
Paragon Home Inspections, LLC