When you have no clue how to spell a word, dictation software might help. If your pronunciation is clear, you can ask your phone how to spell a word by asking it to take a note or run a search. I wonder if that's a Google Now command? It would make sense.
The only trouble is when your pronunciation is also uncertain. If you're looking to spell "deteriorate", it won't do any good if you ask "OK, Google, how do you spell deteriate?" A machine might, with some clever programming, be able to suggest a word that sounds kind of similar to what you've said, but it won't necessarily be as good as a human at inferring the word you want to spell.
Mokalus of Borg
PS - The usual caveats with dictation software apply.
PPS - Such as being unable to use it in quiet places or fearing looking weird.