After replacing the rear disc brake pads and rotors on my 2005 Pontiac Grand Prix, I'm wondering how much drag there should be when turning the rotor?

If there is a noticeable amount of drag, your parking brake mechanism might be frozen in the caliper. That's what happened to mine, the seal on the back wore out and water got inside and rusted it. I replaced both rear calipers and the parking brake cables, which fixed the problem. To test it, jack up the car and try to turn the wheel with and without the parking brake on. If it turns easily without the parking brake and won't budge with the parking brake on, it's probably fine. You can also get an infrared temperature gauge to compare the temperature of each rotor after driving around, or touch them, but you might burn yourself. When my rear caliper locked up, the one rotor was much hotter than the other three.