Really? I must have missed it then. I have flown out of Changi hundreds of times, and have only ever seen what the OP was referring to with a Jetstar BP (i.e. luggage checks at the point of entering immigration area, prior to going airside).

At Changi, I have seen it at checkin on various carriers, although have never been asked myself. I have also seen post check-in, but pre-immigration checks flying SQ at MEL.