Yes, a free-spinning propeller would still provide some drag even if it weren't turning an engine, but the drag would be considerably less.
The information everyone wants is in
NACA report 464 "NEGATIVE THRUST AND TORQUE CHARACTERISTICS OF AN ADJUSTABLE-PITCH METAL PROPELLER" (from way back in the 1930s) which examined this question in detail. Here is the resulting drag at 75 mph for the different configurations tested (the table contains other wind speeds as well):
Propeller stopped:
17° pitch: 53.0 lb drag
80° pitch (feathered): 3.3 lb drag
Propeller spinning (17° pitch):
Free spinning: 33.7 lb drag
Turning a stopped engine: 68.6 lb drag
Engine at idle (power for 350 rpm): 60 lb drag
One important point here is that the stopped, unfeathered propeller is producing almost as much drag as the one that's still windmilling and driving the engine, so it's really not worth pulling the plane almost into a stall to stop the prop for extra gliding range.