For my own code I probably could but I'm more concerned about existing header files (i.e. someone else's files) which might be conditionally including stuff, depending on whether DEBUG is or isn't defined. They probably won't have been written with the assumption that one particular module will define DEBUG - even when all the others don't.

Of course, if the author's correct (in saying that the compiler itself will optimize his macros away) then I'm worrying unnecessarily - but I'm not really sure if that's true.