do you need permission from a photographer/model/company before posting before/after samples in a portfolio?

someone told me that as long as i note the photographer as the copyright then i shouldnt feel "guilty" about using images in my retouching book.
others disagree.

and ironically all the photographers i meet with want to see online samples of work ive done in the past.

anyone know what the legal method is?

You always need permission to use someone else's work. This goes for music, writing, film, and of course retouching.

i thought it fell under the "fair use" clause of copywrite laws, but i've been told i am wrong..it's kind of a grey area as the copywrite laws refer to authors and the written word...

Not gray at all. In fact, the only confusion is caused by those that wish there was some confusion. Fair use only applies to scholarly and editorial use. Portfolios are neither.

what category do Portfolios fall under, as part of fair use allows exemptions for the purpose of critiquing...

retouching falls under the derivative works clause, and as such still needs permission from the original author.

http://www.copyright.gov/circs/circ14.html#derivative/

If the copyright owner disagrees with your interpretation of fair use --- you may get sued. I think the following webpage explains the problems of claiming fair use.

http://fairuse.stanford.edu/Copyright_and_Fair_Use_Overview/chapter9/9-d.html