Some brakes have a more positive disengagement via a spring, but the reality is that incidental skimming contact doesn't cost a meaningful amount of drag. Friction is proportional to the force holding two surfaces together. At zero force, even though there may be some contact there's zero friction. The first revolution or two of the hub pushes the shoes inward so they either don't touch at all, or just skim the highest (lowest) point as the shell spins.

So yes there's some vestigial contact and a trace of friction, but in the scheme of things it's close enough to zero that it's meaningless compared to the other causes of drag on a bicycle.