Nope you do not have to cut it off the cob first. I clean the corn by removing the strings and such. Blanch for a few minutes in boiling water then flash cool in cold water. Store in freezer bags.
I have frozen uncleaned corn still in the husk because we had so much. I just stuck it in a freezer bag. I thawed it, cleaned it and boiled it and it was fine.
If you have silver queen corn you can cream it, blanch it in a bowl in the microwave then pour into freezer boxes. Makes some awesome cream corn.