Some mechanics believe that marking one's tools puts one in jeopardy in the event a tool is inadvertantly left in one's work. Some also believe that the value of the tool is diminished by engraving or marking it.
There's some truth to both. However, I believe in marking my tools by engraving them, and I believe strongly in tool control. I shadow my tool boxes, which involves lining the boxes with foam, and the foam being cut out in the shape of the tool. I'm just shy of fanatical about tool control; I use multi colored foam so that a missing tool is obvious, along with labels on the foam for each tool so it's identified, and a tool control catalog that identifies the inventory for each rollaway or box, the drawer, and the item.
When work is complete, part of the job is ensuring that every tool is returned to it's place, which can be verified from the job or work order, the inventory, and a visual inspection of the boxes.
This does not ensure that a tool can't be left behind, but it makes a good faith effort to ensure that all tools are accounted during work. If someone else borrows a tool, it's got my name on it, and I have a record of who borrowed it to comply with my tool tracking procedures. I've worked in repair stations where we all carried tool chits, which were metalic discs on a ring, numbered and assigned to us. Any time we borrowed a tool from the company tool crib or another mechanic, we'd put a chit there...if a tool gets left behind, trace the tool and you'll find out who left it there. I also believe in taking responsibility for my mistakes.
If a tool is left behind, the aircraft should be grounded until the tool is located. I'm very serious about that. I've been known to put a halt on a fleet if a tool turns up missing, until that tool is accounted.
As far as the value of the tool; I do believe a tool is an investment, but I don't see it as an asset to preserve for later sale. A tool purchase is an investment in that a quality tool should be built to last a long time, hopefully a lifetime, and I'm investing today in the purchase of a tool that will be in use decades later. My tools bear scratches and marks and wear from use, and most are engraved in some place. In some cases, mainly plated tools, the plating may be damaged and may chip or peel as a result of engraving, but other marking such as laser marking may be used.
Where marking tools really comes to value is when working alongside other mechanics. Preventing disagreements over who owns a tool or who is responsible for a tool is necessary. More than once I've been able to settle a disagreement over a tool in someone's box by locating my markings on it. I've had flashlights walk off and located them in someone else's posession. Any claim to it is quickly settled by referring to my marks.
Some don't believe in marking, and they're entitled to their point of view. However, my opinion is that generally those who hold those views don't exercise proper tool control in general. I'm sure you'll hear altenative views on the subject.