Depends entirely on what you have on your C and other drives. In general, since most programs and document folders are by default in the C drive, yes. But if you're in a habit of installing things on other drives and/or actively saving, deleting, moving and in general just changing stuff around then it's just as important and useful to defrag those drives as well.
But since other drives are generally used for backup/more permanent storage it's almost always more useful to defrag your C drive.