Chuck
Most flying schools instructing on the Cessna 150 where I have seen them fill the tanks and then seen two 200 ib plus guys jump in are overweight and it happens all the time.
The ironic thing is that the same instructor runs through WandB calculations and then turns a blind eye.
Having flown a limited amount of ferries I also know with ferry tanks fitted they can be way over grosse weight at weights which would shock the average PPL.
Other than the insurance factor and runway length I would be more concerned with C of G which is an absolute NO NO! Oh of course if its a twin overweight you are more likely to be going down so better treat it as a single.
Pace