I would only do it if the user directly calls ```FitsWCS```.  And I'm not sure about your question about what would be more annoying, but I am inclined to think that NOT getting a warning when you think you're using the right WCS could be more annoying.  Um, maybe if you wanted to be really nice you could add a ```suppress_warning``` keyword (for users who might be about to read in a load of files for which they don't care about the WCS in the header)?