See
http://sbarnhill.mvps.org/WordFAQs/MasterSpellCheck.htm.
For what it's worth, earlier I preferred to turn off the automatic spellchecking, just like you. But it's actually easier to keep it on and let it flag potentially misspelled words. It reduces the risk that I miss an error and send a document to others with a spelling error in it. If it flags a few words that aren't really misspelled, that isn't a big problem. If many words are flagged, such as names, you can tell Word not to spellcheck those.