This is the only point at which I've had my cabin bags weighed by Comair. But I seldom have a wheelie and usually only carry on a bag that would fit under the seat, so I don't have a lot of data points to offer.

One thing that I have, however, seen is lots of wheelies being pounced on for gate checking without them being weighed as such. This used to be no problem on Comair (and positively welcomed by many passengers) because the gate-checked bags would be returned to the aircraft door on arrival. However, AIUI Comair no longer does that and the bags go onto the belt.

Still, if you surrender your bag at the aircraft door on boarding, it does improve the chances that it will be loaded onto the correct aircraft.