Agreed you don't need any pressure holding down the wheel. But you do need it closed. Having the bike swing out of the tray, held only by the fork mount is not a good thing. Not that I have personal experience or anything...
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.