Yes, Common Law marks extends across the US even though they are not federally registered. The benefits of federally trademark is to establish a proof of ownership of the mark. A common law mark will have to show evidences like invoices, marketing material and will have to convince a judge that you own it. Trademark registration certificate, on the other hand is a 100% proof that you are the owner. Secondly, the trademark law gives you additional benefits if you register your mark. You obey the law, the law protects you.