I use my D5100 (and D3200) all the time with manual focus lenses. I don't have a problem in good light. I took this photo in the shade with the 105mm f2.5P at f5.6. I was able to focus on the clear image of the lilies stamen without relying on the rangefinder dot. I cropped off about 25% of the full frame.
In poor light the rangefinder dot is necessary. In very poor light the dot will flicker, disappear and be hard to focus. But generally that's poorer light that I take photos in.
On a tripod in any light the articulating screen in Liveview and magnified will get your focus dead on.