I think it is a totally personal thing. Cropping to less than a full head has always been done. It seems it was quite fashionable in the 60s/70s and that's the thing, it is a stylistic represenation which may or may not work.

See:

http://www.npg.org.uk/live/photographers.asp

and look at photographers like David Bailey and Gered Mankowitz

http://www.mankowitz.com/frmsetmain....troduction.htm

http://www.bbc.co.uk/arts/multimedia/sixties/