Wow it is fascinating to read the huge range of opinions and responses on this topic.
I'm in the "sometimes gift" camp.
I started doing it when flying with my 3 kids. I still do that, but occasionally when I'm flying alone.
It's always a medium (one pound or so) duty free box of chocolates. I usually wander back to the galley after the first meal has been cleared and give it to whoever is back there.
I've never had a negative reaction. I just explain that it's something for the crew to share and enjoy. I don't think I've ever experienced any special attention or service in return, nor has any been expected.
I fly a fair amount (more than most, less that a lot on this forum) and make every effort to treat the crews of the planes I'm on as people. I also know that people can be pains in the ... and try not to be one of those.
The comments about safety and the risks of adulterated treats being given to crew baffle me.