One of the great things about the English language is that it allows you to spell some words in multiple ways. But it’s kinda a good idea to stick to one spelling. Take this example on the Yahoo! front page:
That’s the preferred plural of tornado. But this spelling is also acceptable, though not the first choice of the editors of the American Heritage Dictionary:
It really looks sloppy and amateurish to use both spellings. That’s why you should select a single dictionary as your authority for spelling and always use the preferred spelling.