Some of these cleanouts haven't been used in years, and are ABS plastic fittings. If the plug doesn't come loose easily don't force it, you may find yourself repairing a broken fitting.
Also, at least around here, there are typically two cleanouts right next to each other. The one closest to the house angles toward the street, and the one closest to the street angles toward the house. That way a plumber using a snake can reach every part of the piping. The point is - if there are two, use the one closest to the house, as it naturally drains toward the street.