If you aren't the best typist, but you get close enough for OS X to detect when you have made an error, you can use this to your advantage. I just continue to type, errors and all, and then edit the errors when I am done typing my document.
Your misspelled words will have the red squiggly lines under them. To correct then, simply right click on the word (or control click) and then select the correct word or spelling. I find it speeds up my typing and writing a lot, since I don't have to go back and correct each word and lose my train of thought.