depends on the medium, if it's an online portfolio then you are effectivley (and legally) publishing it, and you don't have the copyright to do so - the company in question may not want links to your company (free advertising or not) - and they are well within their rights to refuse it's use. You should ask when you are doing the job if you will be allowed to use it in your portfolio or not (this includes the work you produce - you don't have the copyright to that either)