Subs are just a name for the component images that you stack to get your final image.
To extend what Humayun says, about the only need I can see for bias frame subtraction outside of what you are already getting with your dark frames would be if you were creating synthetic darks from a master dark library, but that is highly unlikely given what software is currently capable of in this regard
Yes, I've been working on dark frame theory and software in my spare time. Being unemployed has certain (limited) advantages.
Regards,
Eric