Jack, I also work with the camera in manual exposure mode most of the time. I often use a handheld meter in incident mode to measure the light falling on the subject and transfer that reading to the camera's settings. While I use the hand held meter for the specific measurements that I transfer to the camera, though, I use the camera's built-in meter to clue me in on possible changes of the light level from time to time. When the built-in meter shifts, I take another reading with the incident meter.
Why do I use the handheld meter? It's because the camera's built-in meter is often fooled by the color(s) or reflective characteristic(s) of the subject. An incident meter is not affected by the subject at all. You merely need to read light that is falling on the subject or light that is the same as that falling on the subject (light from the sky, for example, assuming sky light falling on your position is the same as that falling on the subject).