Frost IS ice and therefore should be removed before flight (737's do have slight differences, as in acceptable 'areas', maybe the 767 does too!)
The upper surface of a wing is the most critical and must be clean, the underside may have up to 3mm, often caused by cold soaking at altitude followed by re-fuelling.
If there is any doubt then de-ice (even at the expense of a slight delay)
Rgds
CL747