Yes, if the trademark is used only within one state (and thus doesn't qualify for federal registration), state registration is a good idea. State registration, however, does not offer the same level of protection provided by federal law.
The main benefit of state registration is that it notifies anyone who checks the state's list that the mark is owned by the registrant. This fact will lead most would-be users of the same trademark to choose another one rather than risk a legal dispute with the registered mark's owner.
If the mark is also federally registered, a would-be user of the same trademark is presumed to know the trademark is federally registered, and state registration isn't necessary.
For extensive information on trademarks, see Trademark: Legal Care for Your Business & Product Name, by Stephen Elias and Richard Stim (Nolo), which provides the most current information on domain names, changes to trademark statutes and case law, and the latest registration processes.