Witnessing the event got me thinking about this. If you want to thank the crew, go out of your way to thank them. If you really want it to be known, submit the compliment online for feedback. That (I'm told) gets passed on to the crew, but also alerts leadership too.
On QF66 I had an amazing crew. Obviously the CSM came round prior to landing (WP in J). I thanked her personally but also praised her crew. I also let her know I'd be submitting online positive feedback as well, which I have done.
The more I think about it, any gifting prior to knowing how service is going to be is likely to be seen as some form of inducement. I'm fully aware it might not be intended but you wouldn't do that in any other business, so why do it for a flight.