Not sure I'm an expert, but I'd venture to say that if there is no WC code entered, then they don't get a calculation and you'll get dinged in an insurance audit.  You should check with your insurance company to confirm.

There should be a WC rate table set up in Payroll (PR-Setup-Other-Cost Code WC Tables). This table pulls the WC code based on the cost code that is used when entering time.

I always put a default WC code in the employee set up just in case a cost code is missing from the table or a field person is coded to overhead.