When it comes to re-naming pre-mades sims? i leave them alone.
When it comes to marriage? MC assigns those names at the start of each new day in game. On occasion i set the marriage name of the female sim which would of been done by MC anyway.
When there is nothing going on outside of marriage though i don't change any names, i consider it a bit disrespectful to co-opt others work and try to make them mine by changing those names. Besides that the names they have is unique to each sim and to change that is like taking away bacon and eggs from a fry up in the mornings and replacing those with liver pate and mushed carrots, it doesn't fit together or look right.
besides pre-mades and your own sims, the game generates random townies, but i don't bother with those either.
