I don't think there's any hard and fast rule. Assuming the author hasn't declared a title already somewhere, if the books in the series have a single character or place in common, that's usually what we go with first. Barring that, you can certainly name a series after the first book.
1. The Boys of San Joaquin
2. Probably the World's Best Story About a Dog and the Girl Who Loved Me
3. It Was September When We Ran Away the First Time
Thanks!