Not only the door jams, but the underside of the doors, the inside edges of the trunk, and inside the wheel wells.
You'd be surprised how much gunk is inside the wheel wells just sitting against painted metal. [I don't care about dirt on the plastic wheel well liners.] You won't find it on all cars, it's not a problem on my BMW for example, but it definitely is on my Honda. Corrosion from chronic gunk seems to be a problem on light duty trucks too as they often rust just above the rear wheel wells. If you want to check for gunk, curl your fingers around the upper edge of your rear wheel wells - can you touch metal, is it clean or not?