The fair use defense to trademark infringement means that your clients can "nominatively" refer to something even if it's another owner's trademark in order to accurately identify the product or service. For your clients to use Yahoo's and Google's trademarks, there has to be an effort to use only as much as is needed to identify Yahoo's and Google's search engine services, while avoiding leading any consumers to think that your clients' products are afiliated or sponsored by Yahoo and Google. Sometimes this can be done by adding a disclaimer.
But in this case, I think everyone knows that Yahoo and Google are search engines, so your clients should be able to include links to Yahoo's and Google's search services, along with their logos, without causing any confusion.
You're best off consulting a qualified IP lawyer to evaluate your intended uses and help you make good business decisions about product content, advertising, protecting your work, etc.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
Question: My question is, can I advise my clients to use the Google and Yahoo logos in their print ads instead of just the words, Google and Yahoo?
Answer: I'm confused. Neither Google nor Yahoo! use any logos as trademarks. Their trademarks are GOOGLE and YAHOO! respectively. While one of the YAHOO! marks has its letters off kilter a bit, it is still just a word mark. In print advertisements, it's perfectly acceptable to include a statement such as: "You can find us on the web by using Google to search for XYZ Company." It is NOT acceptable to include a statement such as: "You can find us on the web by googling us at XYZ Company."
Question: Could I also use the logos, in that context, on their websites?
Answer: Both Google and Yahoo! have procedures to follow if a website owner wants to include (for free) either search engine's functionality on their website. See . If the website does not want the functionality but merely wants to use the trademarks Google or Yahoo!, then follow the sound advice provided by Ms. Koslyn.