Very likely it's forbidden at some companies, but last time I left a job (IBM UK), DBANning all one's computers was a step on the official checklist. It seems to be a good idea for any company that allows its employees to use work computers for personal web browsing etc., as the employee then takes responsibility for his own private data, rather than trusting the company's IT admins to deal with it.
I didn't. I nuked my personal stuff, and then worked hard to put the machine in a state that my replacements would find useful to reproduce my processes.
I wouldn't.