What’s that old adage: a picture is worth a thousand words? So how cool would it be to have Apple’s logo on your site, or Windows, Coke or Pepsi’s? Few things resonate as well as a high-profile brand on your site. It immediately suggests you have high-profile clients. Or, it could suggest you work for the competition; or you don’t respect copyright. Unless you have specific sponsors, long-term clients, or want to support a particular brand, it may not make sense to add a high-profile logo to your site (other than yours). If you have that close a relationship with a particular organization or company, well at least be sure to get written permission for use of that logo on your site.
Generally, that company will want to make sure your site doesn’t have offensive or other material it doesn’t want to be associated with. Once you have that permission, it’s always a good idea to add “XYZ logo courtesy of XYZ Ltd.” It tells other potential clients you have a good relationship with XYZ and respect their brand. On the other hand, XYZ might also want you to add a disclaimer that states it’s not associated with you nor responsible for the content of your site. That kind of disclaimer isn’t necessarily a problem, but it also doesn’t scream “great talent-client relationship.”
Bottom line: adding your client’s logo to your site isn’t always good; and it isn’t automatically okay.