I ride on the street, sidewalks, alleys, bike trails, driveways, parkways and dirt trails, across grass and gravel, through parking lots and small bodies of water on occasion, whatever makes the most sense at the time. While I probably ride 98% of the time on the street or bike path, sometimes the safest (or convenient) way is to take a detour off the street. If you are not getting in anyone's way, I don't see what the problem is. My commute goes through industrial areas and past cemeteries where there are almost no people on the sidewalks and very little chance of cars pulling out of driveways. If I do resort to a sidewalk with people on it I will slow down to walking speed, but usually stay on the bike.