I think a transition of some sort between different materials usually results in a better looking wall than no transition, much of that depends on how similar or dissimilar the adjacent materials will be.

A small pencil or some other liner can help the eye and the brain more easily move across the wall, and it often sets accentuates the components of the wall instead of having them muddle one another.

While there are things I like and prefer, we don't always see things the same.

Glass next to glass might transition just fine, glass next to ceramic might look better with a liner.

Color transitions can be a different animal, to me "it depends" on the materials and the colors.

Transition pieces can also help when adjacent materials are of a different thickness. They can also help when they are different sizes or set in different patterns.

It really is in the eye of the beholder. But what I've found is that as in your case, if "it doesn't look right", then the homeowner knows that something else is needed in the design, but they just don't know how to make the leap.

This may sound silly, but you want transitions to look planned and not haphazard, and the material acting as the transition piece should be subservient to the other materials. Meaning it should compliment them, not dominate them.

The best I can offer is to "work on it" and get something that is appealing to you.

As to the ceiling, if the planned pattern seems too "matchy" or not "matchy enough" then sometimes it can help to take a different path. Herringbone for a subway, or a diagonal for a square or rectangular tiles, etc.

Ambiguously,

Mongo

Mongo, Thank you so much. You are right, it just didn't seem right to me to put the mosaics right up against the subways. I will do some sort of liner on either side. Also, I love the idea of the herringbone ceiling pattern. Thanks again for this advice and the humor you provide. I always get a good chuckle when I read your other responses on the forum! Cate