I always write down which units are in which transport - but won't tell my opponent. If they ask I just tell them to wait and see
Well maybe some people are reading this the wrong way. In my group yes, we tell you something is in a transport... just not which transport. AKA my plays SW with 2 squads of greyhunters in rhinos and 1 squad of bloodclaws in a rhino, he says "all of 3 squads are each in a rhino." but doesn't tell me squad #1 is in rhino A, etc etc...
Write down wich squad is in what transport before the game but dont tell the oponent. That way there is no cheating and some realism is preserved.
Just to clarify, it is on page 92, where deployment of forces is discussed:
"A note on secrecy
To keep things fair, you should always allow your opponent to read your force roster after the game. In the same spirit, always make clear to your opponent which squads are embarked in which transport vehicle."
To try and trick an opponent, and refuse to state what units are in what transport, is cheating.