Never got charged for oversize with it domestically.The thing with this (and road cases) is that on European flights you can be charged both oversized and overweight fees which can often be 100 Euro or more EACH (if it is both) and are often applied to each flight if there are connecting flights.
EDIT the roto cases would only be oversize. They are not heavy. This is probably what I would use for a neck-through or set neck bass - but would be prepared for the up-charges.