Do we have to register our mark to protect it in the U.S.?

Theoretically no. Trademark rights in the U.S. can be protected under common law without a registration. However, to enforce these common-law rights, it usually requires a large expenditure of money to prove and establish common-law ownership. In dollars and cents, it is more practical to obtain a federal registration for your mark. Should you have to protect your mark, this registration is proof that the U.S. Trademark Office has determined that you own the mark.