I have a few tools with others' names on them. One of them I found in a gutter, and some others I bought at a yard sale from the guy whose name is on them.
What I don't like is when someone marks tools by grinding. I have a few with notches ground in them, and one wrench with "rings" ground around the handle. I think this destroys the value, not only from an aesthetic standpoint, but causes an issue with the strength of the tool as well.
I did stamp my SS# into a few of my SO wrenches many years ago (until I dulled the stamps after about 4 wrenches lol). I now wish I hadn't done that, but what's done is done.
Things I am concerend about being stolen on a daily basis I usually mark in an inconspicuous spot with my DL number. I have it written on a piece of paper rolled up and stuck inside ladder rungs, and have marked some power tools on the inside of the housings or areas covered by guards, batteries, etc.
Many people grind off other names, but I leave them on the few I have with them. I figure it shows I'm not trying to hide anything about their origin.