SQ crews seem to have a habit of asking me to complete a feedback form. I'm usually quite friendly with the crew (well, more so than the other pax which isn't saying much) so I think it's definitely an easy win for them.
One flight comes to mind from a year ago coming back from HND. I seemed to become friends with the entire J cabin crew, whom I'd spent a fair amount of the flight chatting to. About an hour from landing, they dropped in the question of whether I could complete a quick survey. Of course I did, and I life some well deserved positive feedback.
Shortly after, each of the crew came by, one-by-one, to say thank you for the kind words and wished me a pleasant onward journey. They'd obviously all had a read of it!
I'd actually quite like it if they left some forms in your seat so that you could do it without prompting. I love giving feedback when service has been ace, and it's so nice to show your appreciation and give it directly to the crew.