Incident light metering seems the most logic thing to do, the less error prone, also when using colour slide film.

When using reflected light metering you must "place" the subject's skin in some zone, i.e. you must figure out how different is the zone you are measuring from "middle grey" and act consequently.

When using incident metering you don't use the reading as "zone V". You use the reading - period. If the subject's skin - or the thing you are photographing - is dark it will come out in zone IV or III, which is probably what you want, and if it is bright it will come out in zone VI or VII, which again is probably what you want.

Incident metering lets you abstract from "placing". The reflectivity itself of the subject will take care of the "placing". That's the normal case, when you don't want to reach some special effect.

The only thing you must be aware when using incident metering is that when you use slide film and when you have a subject with an extreme subject brightness range you might have problems because the extremes of the range might fall in a zone of the film where you don't have much detail*.

Normally when doing portraits you don't have a very extended subject brightness range. That means you can just take an incident reading, apply that to the camera, and that's going to work quite fine.

When using negative film you have ample "slack room" in the highlights, and not so ample in the shadows, so in case of doubt just open a bit more.

* For instance if your subject is very white somewhere and you want detail in that very white - imagine a bright white wedding dress or a very white hat and you obviously want to preserve the texture of the hat - you should expose a bit less than what the incident metering suggests, when using slides, so that the hat doesn't come out in the upper part of the curve where the detail is mostly gone. If you use an "accent light" you should be aware of the "mirror effect" the light can have on the hat, or maybe even the hair of the subject, and you should avoid using digital technology as that will easily create a white "hole" in the head of the subject