Searching the United States Patent and Trademark Office for a conflicting mark is a must for every business because of the nationwide priority given to federally registered marks. A business does not want to be in a position of having to rebrand years down the road because it did not search the USPTO before investing in a brand. Searching beyond this and to what extent really depends on the business and industry in the business is in.
If the business operates in a saturated market, like the restaurant or craft beer industries, then conducting a comprehensive search for common law (i.e., unregistered) trademarks may be important provided the business aspires to expand throughout the nation. If the business does not aspire to expand, then an Internet search for an identical mark may be sufficient.
However, if a business does not operate in a saturated market, a comprehensive search may not be necessary. More likely than not, the business will know who its competitors are and what marks are already out there. An Internet search may be sufficient in this scenario.
It is important even at the early, search stage to not lose sight of the business objectives with adopting and registering a trademark.