Since most writing is done with spell-check available, knowing exactly how to spell tricky words such as "Mississippi" is no longer important (the program will tell you), but you need to be able to distinguish between, for example, "lose" and "loose", "affect" and "effect", "its" and "it's". I see the wrong word used in these pairs very often.
"To see what is in front of one's nose needs a constant struggle." - George Orwell