Absolutely you should. It’s not just common sense from a security POV, it should also be practice as matter of business ethics.
Let’s imagine the following scenario: Alice leaves, and her computer is transferred to Bob. Bob didn’t know it, but Alice was into illegal shota porn and left several files tucked away outside of her profile. IT wipes her profile and nothing else, which included only her browsing history and local files.
One day, Bob is checking out the bells and whistles on his shiny new work machine, while sitting at a Starbucks™ and sipping at a latte. He stumbles across Alice’s cache and innocently clicks on a file that looks strange. Suddenly, every head in the store whips around to watch in horror as Bob’s PC flouts several state and federal regulations at full volume. One little girl in the corner starts crying.
Bob is mortified. After six months of depression and after having been fired for his unintentional act of public indecency (and possible criminal charges), he finds himself a really crackin’ legal team and lays waste to his former employer with an outrageously damaging lawsuit. Alice is in Thailand and escapes extradition.
Maybe all this is a little beyond the pale, but it absolutely could happen if you don’t take the time to scour through a former employee’s every action. Or you could save time, and reinstall from scratch.