Making it you default will only case it to be used if you open a document by double clicking on it rather than opening it from within, say, Word.
You will still be able to use whichever program you like to produce your document by opening the program of your choice. Personally I have MS Works (Word, etc.) as my default but also have Open Office installed as it suits my needs. I can open either by clicking on its icon but if I open any document sent it will open in Word.