A hardy annual from our Homesitters: “In this enlightened age, should I address clients by their Christian names?”
Answer:  Only if they ask you to do so, or when not doing so would be seen as a rebuff.

We still subscribe to the view that you need to know people a little, before being on Christian name terms.  Skipping the introductory phase may be seen as familiar or even impertinent.  Dogs understand this very well.  Hence the careful anatomical sniffing on first acquaintance.  However, generally we don’t advise our Homesitters to imitate canine behaviour too slavishly.

Incidentally, nowadays there is a school of thought which considers “forenames” to be the approved terminology…