I have a bathroom (not installed by me) which is lined with Travertine
tiles. Due to some plumbing problems, I've had to break a few around
the bath and they're now being replaced. Of course, this has uncovered
other problems - my tiles have not been sealed, and apparently with
porous rock this is a bad idea since it lets steam through to soften
the tile adhesive.

There are also other opinions:

http://www.marblecleaning.org/phpkb/question.php?ID=85

My tiler said he would seal all the tiles for an extra £50, which
doesn't seem like very much but it's a lot when you're already laying
out for everything else and it may not be necessary. Any thoughts?

Peter