Had passengers weighed twice in my career - on both occasions actual weight was below the weight we would have been using had we used the official assumed weights. This was fortuitous since we were on fuel critical sectors which meant we could load some extra fuel to avoid a technical stop for fuel.
On the other hand when I first flew A320 in 1990 we would sometimes get airborne and we would get a "Check Gross Wt" message as the system had detected that we were heavier than the entered ZFW plus fuel etc. We were using assumed weights for hold baggage. The company decided to weigh all hold baggage and we discovered that we were indeed heavier!
That said in this day and age I would have thought some bright spark would come up with a way of automatically displaying (whilst on the ground) actual weight and CG position which was accurate. Am aware that such systems are in place on cargo a/c as a gross error check.