I've tried it both ways, but there's only been a couple of portraits that I actually thought came out decent (More about my lack of painting skills than my masking technique). One was more along the lines of the freehand shield method, and the other I used a cut out to protect the back ground. The freehand one had a much softer outline and had a completely different effect than the hard, in your face, outline I got with the cut out. Both worked for their particular pictures, but both looked much different from the other. There's a ton of great portrait painters on here, so I'm sure, with their help, you'll get it figured out, but just trying out different methods and getting the experience in seeing how each one works for you and the different looks you can get will help too.