I only know anything about the English ones. For them, my attitude was that the number/letter is their historical name (at least to us now) rather than a behind-the-scenes ID, and I wanted to treat it as a name. (Sometimes I had to give precedence to a modern use, e.g. museum name.)

I like it that the letter/number might be rendered as a name on the map. Since it’s a linear spatial sequence (with some missing) it helps to indicate where in the chain you are and where others may be. For people who don’t know the history, it might draw them in to find out that a Martello doesn’t exist in isolation. If the letter/number were only in a ref tag, then not only would it be unlikely to be rendered but a general purpose international map might reasonably not include this data at all.

But then, I’m aware that I have a bias to tag in a way that will be most useful to people who use maps the way I do!

A couple of years ago I did some casual research and tried to make sure that all of the surviving Martellos along the South and East coasts of England were on OSM and named with their original reference number/letter(s). I also added any other relevant info that I randomly gleaned, but I changed as little as possible of whatever tagging was already done, and it’s really nice to see other people making it better and more consistent.

The best resource I found was Peter Faulkner’s website. It seems to be down at the moment but there’s an archived version from May here - it was a mine of information, though it looks like the pictures don’t work in the archive.

All the best!