I also find at least on non LCCs, including SQ, they tend to ignore anything you are carrying on your back. Whilst they may weigh the wheel-aboard, or are busy weighing other wheel-aboards, they usually don't see or don't bother with the backpack that I am carrying (unless I plonk it down on the ground during check in).
And When SIN bound I will often check in luggage even if I'd hand carried it outbound, saves hassle of carting around an airport (I'm thinking in particular BKK) and worrying about OH locker space when 9 times out of 10 the wait for luggage is <5 minutes at Changi.