If a client hires you to photograph for them they own the copyright unless they assign it to you in writing and you are required by law to seek their permission to use any shots of them or their event in your portfolio.
It's that simple.
Even if you own the copyright it's poor form not to ask permission.
i have an understanding with my coporate clients not to use their pictures of the events until after 6months to 1 year. simply 1) they own the rights 2) sometimes the pictures may be sensitive to public scrutiny. same goes for wedding photos. i will ask permission if the couple is comfortable to post it online.