Ebb:

Does your engine have a flushing port for a garden hose attachment? ie flush port at the top of the engine block. If so then you could flush it in any position because you don't have to run it during the flush. If on the other hand you are relying on rabbit ears (flushing from the bottom of the lower unit) then I would not flush in a tilted position because to get full effect of the flush you need to run the engine. I would worry about the problems with the oil sump/pump getting starved if not run in a vertical manner.

Andrew