In Trello, is there a way to set a default label for every card created in a list?
There's no way to set a default label for cards created in a list. You might try the keyboard shortcuts to add them quickly, though. https://trello.com/shortcuts
There is one "way": you can create a board named "Template" in your personal or organization board, and copy it for every new board. Then you only have to modify it.