At the moment, I see 85 questions tagged nginx, while Serverfault has 6,775. StackOverflow has double that number.
1 Answer
As webmaster use webservers, which are servers, there's always an overlap between this site and that. If it's a question which a webmaster needs to know the answer to, and the technology they happen to be using is nginx then it probably belongs here. If it's a more detailed question that a server expert would ask then it probably belongs there. Choose one site and if it's in appropriate it will get moved. You won't get penalised.