Only stupid questions are the ones you DON'T ask.
Either way, open or closed (as per the Thetford Cassette instruction book).
You can run some water into the bowl before use, then open blade to flush OR open the blade before use (my preference), flush after use and close blade. Don't leave water in bowl and don't travel with water in the flush tank (can cause water damage).
Only quoting from the Thetford instruction book that came with my Expanda...repeated word for word below.
Important Warning Notice! Please do not travel with water in the flush tank of your toilet. Failure to adhere to this notice may result in water damage to your caravan or motor home.
Great to know what others do. I think it would be stupid to have to empty the flush tank every time also. Im with CD on this one and believe they are trying to cover themselves.