If you hardly get any rain and have sandy soil, I don't think you have to worry about too much water and root rot. In fact I would think you would probably worry about ithem getting enough moisture.

I had a lavender plant for at least 10 years until the frost finally killed it off last year. I babied that plant(it is not frost hardy in my area) I had it mulched and added more mulch in the winter. I never once thought that I was giving it too much water or wondered about root rot.

We have clay soil that I have amended with organic matter but underneath it is still clay and we have most years normal rainfall. I also watered it during dry spells so it got plenty of water.
Our summertime temperatures range in the 70's and 80's.

Although you apparently do not consider it as such you used the rocks as mulch. The amount of moisture retention depends on the type of rock.

You should always leave some space between the plant and any mulch you apply. The mulch of any kind can act as a wick to remove or to deliver moisture to the stem of the plant. By leaving a space you lessen the wicking.