Framing in camera for print

[rdptheman65]

I have done a lot of study in photography and many times when I'm studying courses such as the Kelby online training courses, instructors will take photos and frame the picture in the camera like they want to see it in print. In my experience, when taking portraits, wedding photos, etc. I don't know when shooting what size print will be requested. With this in mind and since most cameras including my Nikon D700 produce a native 1.5 (4 to 6) ratio, do most photographers who are experienced portrait and wedding shooters leave room in the frame to accommodate all size prints. When I see training videos by pro photographers it would appear they frame their shot in the camera to exactly what they want to see in the picture. If you do this it seems to me you will only be able to do a print in a 4 to 6 ration such as a 4" x 6" photo.
I appreciate any input from photographers experience in this kind of work.