Just thought I'd add my current experiences to this post for an itinerary I booked on the SQ web paying via credit card.
Nov 4 SQ220 SYD-SIN asked to show credit card on check-in
Nov 6 SQ862 SIN-HKG not asked to show card at check-in
Nov 9 SQ863/SQ221 HKG-SIN-SYD not asked to show card at check-in
So much for consistency