For the last five years or so, I've been using a consistent pattern for by characters in terms of what scars and how much.
Each time I take a wound that goes over a certain threshold, I make a note of it on a spreadsheet. Then when I have time, I add appropriate scars. Here's the chart I've been using, for characters with around 100 HP's. Note that this is per hit, not total or total per location.
|6-8||x||maybe light scar|
|9-15||xx||pale, faint-looking scar|
|16-22||xxx||puffy line of scarred tissue|
|23-29||xxxx||long, jagged-looking scar|
|30-54||xxxxx||pitted, deep-looking scar|
This isn't a hard and fast rule, even for me. It's a role-playing aid, designed to add a little more flavor to the character and get me away from shaking off serious wounds. Each character usually starts as a blank canvas, and I work from there.
With the new "new" scars, I need to make some decisions. If the new scar is pretty terrible, I start it as a red scar and after a game year I fade it to a faded scar. And I had to arbitrarily decide that only the worst level of scar will show.
You see that last row, with "Serious injury/loss"? That's when my characters start losing fingers, eyes, ears, hands, etc. Because brutal, terrible things happen sometimes.
[If you're wondering where scars come from, head to a quit-safe room and type "change locdesc" -- it's a great new feature.]