Largely, it depends. Northstar put out a book called Omega in the late eighties and were promptly slapped by a suit from Marvel over the Omega name. (Despite the fact that Marvel had not used "Omega the Unknown" for several years at that point and continues to not use him.) Northstar was legally forced to change the name of the book to Omen. (They even printed the notice from Marvel in the back of the book, a nice little Romita Sr. Spider-man on the bottom of the page.)
