@Buttery_Mess:
Most of the time you can cross the train tracks without looking and not get hit by a train too...
As long as the entity whose data you modify isn't moved or removed, you can modify elements of it that are small enough (and suitably aligned) to be updated as indivisible operations without risk (assuming the elements in question are independent, and ignoring the multiple levels of cashing of modern processors). If data is split into multiple indivisible transactions you run the risk of the other process reading data that's partially the old one and partially the new one, which isn't particularly safe if one element is a pointer to an object and the other element specifies what type of object the pointer points to, for instance.
DF retains units in the units list after they've died, so it may be that their data structures are all retained as well until a new save is loaded (which DT detects), in which case modifying jobs on dead units may well be reasonably safe. Making the assumption that all data is safe to manipulate because some carefully selected ones are is rather risky.
Edit:
Response to Shonai_Dweller's next post:
The response to Buttery_Mess' entry did not attempt to comment on iceball3's post. However, unless you're able to travel in time, it's hard to apply sun screen and protective clothing to the children growing up to become Toady and Threetoe, as the main concern is the result of past sun exposure. And I have reason to believe the current versions of these persons do not rely on hair alone to protect their scalps (although the advice may well be a useful reminder for others).