I just replaced my rear brakes last weekend, disks and pads. The pads had a bit of meat left on them, and the disks were within tolerance according to my micrometer, but they pulsated pretty badly when applied at speed....warped disc.
Anyway, sounds like your parking brake is adjusted too tight.
In answer to your question, there will be zero gap between the pads and the disc, which is desirable for minimum brake pedal travel.
How many notches does your parking brake travel when you apply it?
If the disk brake was binding, you'd feel it get tighter through part of the wheel's revolution as you turn it by hand, which would indicate a warped disc, which you'd feel through the brake pedal as you applied them when driving.
Really easy job replacing the rear brakes. The hardest part was having to whack the old disc off the hub, as it was rusted on some. Also, the center of the hub on the disc forms a drum for the parking brakes.
The maintenance procedure actually reccommends you carefully apply the parking brake at speed, say 25 mph, on occassion to clean the parking brake shoes. Could be you have brake dust built up on yours (from the disc brakes), binding up the drum brake.
Honestly, if you still have plenty of brake pad left, and the discs are within tolerance for thickness and runout, then I wouldn't worry much about it, depending on how hard they are dragging.
On the plains of hesitation bleach the bones of countless millions who at the dawn of victory, sat down to wait, and waiting -- died