LDR is just low dynamic range such as your mailbox scene. There are no highlights and no deep shadows. The whole scene can be covered in a single shot.
If you have a scene with any highlights, mid tones and shadows then its a good candidate for HDR and tone mapping. To cover the highlights shoot as many shots as necessary. The same for shadows. 3 shots might be enough for a lot of senarios, but often 3 isnt enough to cover both the brightest and the darkest areas in the image. Really bright highlights like sun or bright lights in a dark scene need more than 1 shot to cover them. The same for the darkest areas. If you have all the range covered and combine the images in Photomatix Pro you should come out with a pretty decent image. Dont play with the sliders so much as selecting a preset you are comfortable with and then tweak with the sliders if necessary. When the image is finished its not. Save it and then finish it off in Photoshop for noise reduction, exposure and gamma correction and finally unsharp mask.